FRCS 3e remains the gold standard for an encyclopedic tome. My only complaint is that there weren't more instances of site adventure maps with an ultra-simplified key, so you didn't feel compelled to hunt down a billion sourcebooks to run adventures in some of these locales. That and leaving the big regional encounter charts for the DM Screen supplement.
Faiths & Pantheons is absolutely incredible. It did a great job of summarizing the deities, making them gameable for clerics, and making it clear how/why they were so involved in the material plane.
Lost Empires of Faerun was really good for making the history of the setting gameable. And for correcting a lot of really dumb naming conventions from late-era 2E supplements.
The 2E setting boxed set was great for the setting maps and hex overlay. Not having hexes on maps is a real, real pain, given its a conceit of the game through nearly every edition.