(1974) OD&D -> Holmes Basic -> 1e (1977)
Within 1e, you see the classics of the genre:
1977 Monster Manual
1978 Player's Handbook
1979 DM's Guide
1980 Deities & Demigods
1981 Fiend Folio
1983 Monster Manual II
1985 Legends & Lore
October 1985 Oriental Adventures
December 1985 Unearthed Arcana
1986 Dungeoneer's Survival Guide
1986 Wilderness Survival Guide
1987 Manual of the Planes
1987 Dragonlance Adventures
1988 Greyhawk Adventures
So, other than Legends & Lore and Greyhawk adventures, I still own at least one copy of each of these. I would place the decline beginning in 1984 myself. MMII was the last of the really good books IMO. However, I got a bit of something out of all the other books except Dragonlance adventures (never used it or played in it).
Of the later books, my favorites were OA (when will 5E make this???), UA, and MotP. The two survival guides were okay and dragonlance was interesting. But unlike many here (apparently) I am in the minority that I LOVE UA! Love the new classes, love more magic items and spells, love it all.