D&D3.5 environmental books
I have all of the D&D3.5 environmental books, but my favorite in terms of unique value is Sandstorm. I've used all of them, but that's the only one without a D20 rival that has more extensive rules (Frostburn/Frost & Fur, Stormwrack/Seafarer's Handbook and various other nautical books, and Cityscape/Cityworks), not to mention books such as the Wilds or Classic Play Book of Adventuring.
They're all decent, but none is as concise or comprehensive as the AD&D "Dungeoneer's Survival Guide" and "Wilderness Survival Guide" were in their day, IMHO. Of course, I could be biased toward the D&D3.5 Sandstorm text, since I'm running a desert-based campaign right now, but I have half a bookshelf's worth of books for just about every other terrain.