Which FR focused sourcebook should I buy?

[Nymrohd]

I pretty much have every book published on FR. Unapproachable East stands among these books as a potentially separate mini setting that can be transplanted anywhere (or played along without consideration for the rest of the realms).
Underdark is a great book. I prefer the 4E underdark book but it was far better than the Drow of the Underdark in late 3.5 and has several interesting rules items you can transplant seemlessly.
If you are looking to mine for ideas, Lost Empires of Faerun is choke full of FR lore and describes dozens of civilizations, yet not with enough detail that you can cut and paste them anywhere.
Shining South was a disappointment, most of it (like a ton of 3E material) is just a reprint from the AD&D original book of the same name; Seprent Kingdoms has great ideas but is too steeped in FR lore and the locations are fragmented.

 

[Desdichado]

That's good to know, actually.

I actually kinda like the Drow of the Underdark 3.5 book, but I have to admit that alongside Green Ronin's Drow book and the Paizo Darklands book, I wonder if I might not be pretty well covered on that theme already.

I certainly don't want stuff that's too steeped in FRiana; that's the whole point, I want stuff that would still be useful, helpful, or at least interesting in any other setting.