While many of the names in the book are identical or close to those in the later versions, some of them bear almost no resemblance to their final forms. J. R. R. Tolkien changed names rather frequently, sometimes with several new variants (rejected in turn) written in a single manuscript. Confusingly, sometimes the name applied to one thing is later used to refer to a different thing, the original use abandoned. As an example, the house of Elves called "Teleri" in The Book of Lost Tales is not the same as that in The Silmarillion (see Teleri). The original usage of "Teleri" would eventually change until the name became "Vanyar". Meanwhile, the house of Elves called "Solosimpi" would inherit the name "Teleri".
The framework for the book is that a mortal Man visits the Isle of Tol Eressëa where the Elves live. In the earlier versions of the "Lost Tales" this man is named Eriol, of some vague north European origin, but in later versions he becomes Ælfwine, an Englishman of the Middle-ages.
There are more changes visible within the book, and it is not internally consistent, partially because even while still writing it Tolkien began rewriting earlier parts as his ideas about the world changed. The Tales were eventually abandoned, but they were resurrected in part as the "Sketch of the Mythology" which would become the Silmarillion.
For publication the book was split into two volumes: The Book of Lost Tales 1 (1983) and The Book of Lost Tales 2 (1984), but this is simply an editorial division. Both volumes are separated into several "Lost Tales".
There is an inscription in the Fëanorian characters (Tengwar, an alphabet Tolkien has devised for High-Elves) in the first pages of every History of Middle-earth volume, written by Christopher Tolkien and describing the contents of the book. The inscription in Book I reads: "This is the first part of the Book of the Lost Tales of Elfinesse which Eriol the Mariner learned from the Elves of Tol Eressëa, the Lonely Isle in the western ocean, and afterwards wrote in the Golden Book of Tavrobel. Herein are told the Tales of Valinor, from the Music of the Ainur to the Exile of the Noldoli and the Hiding of Valinor.".
The inscription in Book II reads: "This is the second part of the Book of the Lost Tales of Elfinesse which Eriol the Mariner learned from the Elves of Tol Eressëa, the Lonely Isle in the western ocean, and afterwards wrote in the Golden Book of Tavrobel. Herein are told the Tales of Beren and Tinúviel, of the Fall of Gondolin and the Necklace of the Dwarves.".