What were the go to Books, aside from the usual edition trinity, to get for 2E?
Definitely the Core Rulebooks plus
Tome of Magic.
As a Forever DM who doesn't like running the same world twice in a row:
Player's Option.
Skills & Powers for races and subraces,
Spells & Magic for priesthoods and magic systems,
Combat & Tactics for era-appropriate weapons and technology.
World Builder's Guidebook and
Complete Priest's Handbook are also critical. Also, consider the
Complete Book of Humanoids.
If you like more Asian-inspired stuff,
The Scarlet Brotherhood (for
Greyhawkupdates the Monk and Assassin classes, the
Complete Ninja's Handbook revises OA's martial arts systems, and you can rebuild the spellcasters with
Spells & Magic. The
Kara-Tur sub-setting for the
Forgotten Realms might be able to flesh this out further, but I've never run with it.
For high-level gaming,
DM's Option: High Level Campaigns is very useful.
Dragon Kings, for
Dark Sun is more useful in other settings than it would appear to be. I can't vouch for the FR high-level stuff.
The encyclopedias (of spells and magic items) aren't terribly inspiring or evocative, but they are incredibly dense-- if you can stomach just sitting and reading the equivalent of a phone book for a few hours, you'll come out of it with a handful of neat ideas.
A lot of the setting-specific stuff can be really useful for homebrewers, but I'm not really in a position to make recommendations-- beyond the
Planewalker's Handbook and
Complete Spacefarer's Handbook.