The Book of Lost Tales is the title of the first two volumes of Christopher Tolkien's 12-volume series The History of Middle-earth in which he analyses the unpublished manuscripts of his father J. R. R. Tolkien.

The Book of Lost Tales contains the first versions of the stories which would later become The Silmarillion, and is notable because, while very primitive in style and content, it is very close to the later work in many ways. Each of the Tales is followed by notes and a detailed commentary by Christopher Tolkien.

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 and The Book of Lost Tales 2, but this is simply an editorial division. Both volumes are separated into several "Lost Tales".

Contents of Book 1:

"The Cottage of Lost Play" — the "framework" story
"The Music of the Ainur" — the first version of what would become the Ainulindalë
"The Coming of the Valar and the Building of Valinor" — later Valaquenta and first chapters of Quenta Silmarillion
"The Chaining of Melko" — Melko is an earlier name of Melkor
"The Coming of the Elves and the Making of Kôr" — Kôr is the later Tirion and its hill Túna
"The Theft of Melko and the Darkening of Valinor"
"The Flight of the Noldoli" — "Noldoli" or "Gnomes" are the Elves later called Ñoldor
"The Tale of the Sun and Moon"
"The Hiding of Valinor"
"Gilfanon's Tale: The Travail of the Noldoli and the Coming of Mankind"

Contents of Book 2:

"The Tale of Tinúviel" — first version of the tale of Beren and Lúthien
"Turambar and the Foalókë" — first version of the Túrin saga
"The Fall of Gondolin" — the only full narrative of the Fall of Gondolin
"The Nauglafring" — a story which was "lost", in that it never was rewritten in full, and was mostly left out of the later Silmarillion.
"The Tale of Eärendel" — the only full narrative of Eärendil's travels
"The History of Eriol or Ælfwine and the End of the Tales" — an essay about the changes in the framework, and the "unwritten" tales.