This post is intended as a previous complement to this one offering a reading guide.
The general reader, with a vague knowledge of Tolkien as a famed author, will know of The Lord of the Rings, his most popular novel, and maybe of The Hobbit.
A reader a bit more acquainted might also know that the latter is more children-oriented than the former, but the former being the sequel, they offer a convenient sequence for a young reader.
That more acquainted reader might also know of other of Tolkien's writings, some more abstruse, some more children-oriented even than The Hobbit, therefore not having a particular interest in further reading Tolkien.
Starting from this point, I want to give an overview of Tolkien's mythological fiction, in order to help to understand it and maybe —or maybe not— spark the interest for an informed reading, in any case without spoiling its contents.
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1. The Book of Lost Tales.
During the 1910s decade, Tolkien created a bunch of fantasy tales, with different inspirations, and bundled together within the frame of an invented mythology. They basically told of the conflict between the race of Elves and the dark power in the north of the world, set in a mythical past and geography of Europe —being in this aspect similar to the setting of contemporary writer Robert E. Howard's Conan the Barbarian tales. Tolkien called that bundle The Book of Lost Tales, and by 1920 it was almost complete.
The declared aim of The Book of Lost Tales was be to serve as a tribute mythology for England, in a similar way as the Kalevala for Finland. At that time, Tolkien could have envisioned its publication in an indefinitely distant future, with time enough to complete it —he was still in his twenties. But, as we will see, he would enter into a series of revisions that would take the rest of his life, and even beyond, in others' hands, never coming to the degree of completion he reached with the Lost Tales.
Tolkien produced as a fantasy author an enormous load of material, which apart from The Hobbit and The Lord of the Rings was mostly left unpublished at his death in 1973. The main work with a more or less "canonical" version of the mythology, The Silmarillion, was published four years later by his son Christopher, who would then go on —and later others commissioned by him— to publish all the manuscripts —most of them reunited in the twelve-volume collection The History of Middle-earth—, with both popular and scholarly intentions, and to that impulse we have to thank that we can have such broad and deep knowledge of his father's production.
Also, language being one of the main motivations in Tolkien's life —he would become a philologist—, it was in the root of his fantasy writings, and he produced several stages of different fictional languages. However, I will here focus on the mythological parts, just pointing out that those languages have found an outlet in the specialised magazines Parma Eldalamberon ("The Book of Elven Tongues") and Vinyar Tengwar ("New Letters").
2. Beginnings of the Silmarillion.
In the late 1910s Tolkien begun a revision of his legends. He started to versify three of those tales, the ones which told of three mortal Men involved in the war between the Elves and the Dark Lord. He would only achieve an advanced versification of two of them, but the three would remain the principal of the mythology.
Also, as an explanation of the frame of those three stories, Tolkien produced an abridged "Sketch of the Mythology" for his friend R. W. Reynolds. That "Sketch" would be the starting point to the revamping of the legends, always with —among others— the motif of turning the previous fairy tale tone into a more serious, epic, and darker one. The name Lost Tales was dropped, and the new one of Silmarillion started to be used instead; that was the name that stuck and finally came to the public light.
In addition to expanding the "Sketch", Tolkien started providing his mythology with works of more diverse kinds, as chronologies, geographies, and etymologies. In the middle of that process, an opportunity appeared for Tolkien to finally publish his mythology —or so he thought—, and it was due to a fortuitous series of events.
3. The Hobbit.
That Tolkien created children's stories for his own was not fortuitous at all —several of them have also been published—, and The Hobbit was one of them. But luck did play a part in the publication of the book, and its success.
The Hobbit is now set well within the mythology, and the reader will find in its pages names of people and places taken from Tolkien's previous but then unpublished epic. However, for a long time —since Tolkien himself stated it— it was believed that in the beginning The Hobbit did not belong to the mythology, and that the appearance of those repeated names was a mere "self-plagiarism". That was the reason why Christopher Tolkien excluded the manuscripts of The Hobbit from being published in The History of Middle-earth, commissioning the task to other scholars instead.
Therefore it was John D. Rateliff who finally brought to light the creation process of The Hobbit, showing that, beyond that self-plagiarism —reaching much deeper levels until becoming an incredible wealth of parallels—, Tolkien had in the beginning explicitly thought to place the tale not long after the tales of his mythology —the explicit struck-out sentence exists.
Moreover —as I have elsewhere argued— directly from Rateliff's work it can be reasonably claimed that, due to the almost exact parallel in the succession of geographical features between the lands in the mythology and those in The Hobbit, the latter was initially intended to take place also in the same lands than the former —the same conclusion is almost reached by such a renowned scholar as John Garth. Which is a very logical thing if we keep in mind that Tolkien was creating The Hobbit at the same time than the map of his mythological lands.
The definitive proof that The Hobbit belonged to the mythology was that, when it became a success and its author was asked by the publisher to produce a sequel, Tolkien saw the opportunity and tried to push the publisher into printing the sequel together with the main mythology.
4. A lost opportunity.
By then the mythology had reached a new, definite stage, called by Christopher Tolkien "early Silmarillion". But Tolkien's gambit was rejected, and the mythology had to wait.
Had Tolkien's proposal been accepted, he would not have needed to complete the outline of the mythology, just to attach The Hobbit to it, defining the chronological and maybe geographical adjustment whose options I also studied —same essay.
The most likely scenario would by then be making The Hobbit one more of the tales of the mythology, maybe by creating an abridged tale of the adventure as an additional chapter to the "Silmarillion".
He would also probably had attached an additional unfinished work, The Lost Road, that on this case, contrary to The Hobbit, had actually started as independent from the mythology, being Tolkien's version of the myth of Atlantis. But The Lost Road had inevitably gravitated towards the mythology, and ultimately received the attention that Christopher Tolkien skimped on The Hobbit, being included in The History of Middle-earth.
All in all, a Silmarillion of, say, 1940, could have been enlarged from Tolkien's plans from 10 years earlier just with two Atlantis and Hobbit-driven chapters, just like it finally happened in 1977. But those two would have been much more epilogic, depicting the conclusion of the mythological times and the transition to the mundane real history right after the end of the war between the Elves and the dark lord.
Instead, Tolkien had to leave aside the "Silmarillion", and then he gave himself freedom regarding the fitting of The Hobbit and its sequel to the mythology. The choices he made produced a dramatically enlarged chronology and geography, changing his fictional world forever.
5. Niggling adrift.
Having finished the sequel to The Hobbit —which of course was The Lord of the Rings—, Tolkien turned again to the mythology. But he would never finish it, which might come as a surprise to anyone knowing of the success of the Rings, one of the most popular books of the 20th century. We can list several causes for that failure, if we may call it that way.
One of the points we must take into account is that the huge success of Rings, and the corresponding incentive to offer the Silmarillion to the public, came in the 60s, too late for Tolkien to face the challenge and force himself into finally completing such a grand endeavour.
Another cause was that, instead of going straight to completing the conclusive chapters of his bunch of tales and taking care of consistency —the task inherited from the possible publication in the 1940s—, he, much in accordance to his way of working, started to drift away in several directions.
On the one hand, he entertained himself in expanding on the ages and regions created by Rings, partly impulsed by the composition of the "Appendices" to The Lord of the Rings.
On the other hand, he embarked in a reboot of the cosmology of his fictional world. For some reason, Tolkien became unsatisfied with the idea of the cosmology and corresponding chronology being also fictional, and wanted instead to make it match more closely the real one —we must not forget that the science of Cosmology had been making dramatic advancements during Tolkien's lifetime.
That reboot provided also a much expanded chronology to the prehistory of the sentient races, thus also making it closer to the evolution of species.
In this point I agree with Carl F. Hostetter that Tolkien could have cut short by saying that the event that turned the mythological flat world into a more real round one had also put other aspects of the fictional reality on the same path.
Had the Rings been a huge success one decade earlier, and Tolkien not being distracted with the minute details he liked to expand on, maybe he could have finished the Silmarillion. But, as the short tales Leaf by Niggle and Smith of Wootton Major show us, he even assumed his parsimony. In them we get an idea of how he contemplated himself in relation to fantasy writing and his lifelong works in that field.
6. Posthumously.
As we have said, it was Christopher Tolkien, his father's literary executor, who undertook the difficult task of assembling a publishable Silmarillion as soon as possible, at a time when the boom of Rings had passed.
Christopher discarded the aforementioned cosmological reboot, though he did not keep some marginal elements from slipping in —v.g. changes in naming and some figures. He faced a bulk of texts with very different levels of development in order to produce a text as homogeneous as possible, and to his credit we can say that, despite some errors he later admitted to have made, he produced a Silmarillion arguably close to what Tolkien sr. could have done had he had some more years and vigour to do it himself.
After that Christopher, confronted with the wealth of texts that could likely be of interest for Tolkien fans, embarked in the publication of unpublished incomplete texts, in a rawer form and with editorial notes and commentaries. First he did that with some texts that he was sure represented his father's final thoughts —therefore presumably in accordance with The Silmarillion— in Unfinished Tales, and then chronologically in The History of Middle-earth, during whose publication he came to know of unexpected difficulties in his father's works, to realise of mistakes he had made in preparing The Silmarillion, and to find that the whole of that was not commercially publishable in the end. So Christopher had to hand over the publication of those "extreme" texts to others.
In The History of Middle-earth we therefore find previous layers and versions of the mithological stories of The Silmarillion together with expansions, both in level of detail and themes, that did not fit it, and even texts that were unknown to Christopher when he composed it.
In later years, Christopher Tolkien wanted to offer the public the old three core stories of the mythology as standalone books, as readable as possible, but only The Children of Húrin (2007) is a homogeneous tale, composed in a similar way to The Silmarillion, while the other two —Beren and Lúthien (2017) and The Fall of Gondolin (2018)— are just a rehash of already published texts.