I haven't read them all as of yet, but those
books/chapters I have read so far I found highly interesting and amusing as well. Like it has been mentioned already they contain different versions of JRR Tolkiens accounts of
the history of Middle Earth from the Creation of
Arda until the 4th age, commented to some degree by Christopher.
Vol 1 & 2 (The Lost TalesI & II) are the predecessor of what has become
the Silmarillion later,
Vol III (The Lays of
Beleriand) contains "The Lay of
the Children of Húrin" and "The Lay of Leithian" (=~Story of
Beren &
Lúthien) on form of Ballades or better Poems,
Vol IV (The Shaping Of
Middle-Earth) describes the "status quo" of the mythology of Middle-Earth by the time the
Hobbit was written
Vol V (The Lost Road And Other Writings) tells about the Fall of
Númenor, and about
Valinor and Middle-Earth before the “Lord of the Rings” and furthermore contains a very interesting chapter about the Etymology of the
Elvish-languages. (I really ought to read that one someday…

)
Vol VI (The Return of the Shadow), Vol VII (The Treason of
Isengard), Vol VIII (The
War of the Ring) and Vol IX (
Sauron Defeated) provide us with the history, development and background of the LotR, plus another Version of “Fall of Númenor”, called “The Drowning of
Anadûnê”
Vol X (
Morgoth’s Ring) and Vol XI (The War of the Jewels) contain the later Versions of the
Tales of the
First Age as they led to the “Silmarillion” and – last not least
Vol XII (
The Peoples of Middle Earth), deals with Prolog and Appendices of the LotR, and a few “Late Writings” of the Prof.