The most essential volumes of HoMe are IV, V, X, XI, and XII. Volumes IV and V --
The Shaping of Middle-earth and
The Lost Road -- contain the earliest sketches of the 'Silmarillion' mythology, including the first Quenta Silmarillion. These lay the framework for the later books, X, XI, and XII --
Morgoth's Ring,
War of the Jewels, and
The Peoples of Middle-earth -- which provide the latest Silmarillion revisions and entirely new, fascinating pieces.
The Books of Lost Tales are good reading, but too obsolete to shed any light on Middle-earth. Still,
The Fall of Gondolin is incredible, as is
The Nauglafring (which is, if I'm not mistaken, the only complete account by JRRT of the Fall of Doriath). Volume III provides lots of fantastic poetry, though they're all unfinished.
Volumes VI through IX are boring as crap.
Here's a useful page:
Mellonath Daeron.
[ July 29, 2002: Message edited by: obloquy ]