Cover of a notebook in which Tolkien wrote part of The Book of Lost Tales (from Tolkien Gateway)
Background
Partially written during J.R.R. Tolkien's service in the First World War, these unfinished works are some of the author's earliest writings. Though Tolkien never published the Lost Tales during his lifetime, his son included and analyzed them in the first two volumes of the History of Middle-earth series.
The titular tales are the first incarnations of myths that Tolkien would later unify into his legendarium. He rewrote and reimagined many of them over the course of half a century, solidifying the world that would give rise to The Hobbit and The Lord of the Rings.
With their archaic, at times inaccessible language, the Lost Tales can seem alienating to first-time readers. Nonetheless, they provide a fascinating look into Tolkien's authorial evolution and are engaging, poignant texts in their own right.