OK, I haven't read this book, but I plan to. Here's a few I know of and they aren't early in their careers. Each artist had their first release five or more years prior to these albums. Every one was a successful act. Don't shoot me on the accuracy of the dates, but they're all close:
Get Back - Beatles 1969 re-tooled in 1970 as Let it Be, but never released in its' original form.
Smile - Beach Boys 1967 released 2012
Human Highway - Crosby, Stills, Nash & Young 1974 still unreleased, unofficial releases exist
Dinosaur Tracks - Tower of Power 1982 released 2005 as a limited edition
Stone of Sisyphus - Chicago 1992 released 2010 as Chicago XXXII
Potatoland - Spirit 1972 released 1981
Thom Bell Sessions (Mama Can't Buy You Love & others) - Elton John 1977 originally rejected by MCA. released as EP in 1979
Lather - Frank Zappa 1977 released 1996
Basement Tapes - Dylan 1967 released 1975 bootlegged since late '60's. created modern bootleg industry
Black Album - Prince 1987 released in 1994
Save for a Rainy Day - Jan & Dean 1967 released 1996
Live at the Garrick - Frank Zappa 1968 still unreleased
Dumb Angel - Brian Wilson late 60's still unreleased, except as unofficlal
Flash Harry - Nilsson 1980 released only as an import, although it was a US record company (Finally released as a US CD in '13)
Gunfight at Carnegie Hall - Phil Ochs 1970 released in Canada in 1974, 1985 in US
Try it Before You Buy It - Michael Bloomfield 1973 released in 1988
Oh, My God - Chambers Brothers 1972 still unreleased
5.1 DVD-A of Chicago X 2004 still unreleased Test pressing done (I have one)
Hot Hits/Cold Cuts (compilation) - Paul McCartney 1975 still unreleased, although several unofficial Cold Cuts packages were released
McCartney II (2 disc version) 1980 excised tracks are mostly instrumental, and far more experimental. Although some made the 2012 ultra deluxe, most have only appeared on unofficial releases
That looks interesting....are most, early releases, in artists careers ?