In it the gradual transition from the "primitive" legendaria of The Book of Lost Tales to what would become The Silmarillion is described, and it contains a text which could be seen as the first "Silmarillion": the "Sketch of the Mythology".
Three very interesting other parts are the Ambarkanta or "Shape of the World", a collection of maps and diagrams of the world described by Tolkien; and the Annals of Valinor and Beleriand, chronological works which started out as timelines but gradually turned into full narrative.
Prose fragments following the Lost Tales — brief, uncompleted texts which continue on from The Book of Lost Tales
The earliest "Silmarillion" — also referred to as the "Sketch of the Mythology", this is the start of the Silmarillion proper
The Quenta — a further developed version of the "Sketch", the first full narrative since the Tales
The first "Silmarillion" map — a reproduction of the first map of Beleriand
The Ambarkanta — cosmological essays, maps, and diagrams
The earliest Annals of Valinor
The earliest Annals of Beleriand
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 IV reads: "Herein are the Quenta Noldorinwa, the History of the Gnomes, the Ambarkanta or Shape of the World by Rúmil, the Annals of Valinor and the Annals of Beleriand by Pengolod, the Wise of Gondolin with maps of the world in the Elder Days and translations made by Ælfwine the Mariner of England into the tongue of his own land.".