Books you didn't expect much out of -bought anyway- and got a lot of use from.

[Nightfall]

Yeah well Psion is a mecha fan. But for me I'll revise my list to add the Faithful and Forsaken. I won't use ALL of it, but there's a lot of good fluff and crunch I'll use.

 

[Harlock]

Rokugan. What a beautiful campaign setting book. All campaign setting books could benefit from looking this great as well as having such a vast wealth of campaign information. What sucks is that it benefitted greatly from having Oriental Adventures already in print to crank out the crunch factor, monsters, etc. Still, a trend toward bigger CS books might make up for that...

Honestly, Creature Collection (unrevised, volume one) was the biggest surprise of all in my d20 period, thusfar. I must say it doesn't honestly meet the criteria set forth in the Original Post, as it was a Christmas gift last year, so I didn't actually have ANY expectations for it, because I'd passed it up in the gamestore for a LONG time. However, having recieved it, I read it. Very glad I did as well, because now I run a Scarred Lands game. The information within that monster book had so much flavor for the campaign world that I ended up buying every Scarred Lands book I have seen since. Hell, I own more of them than Nightfall does.

 

[Nightfall]

Just two more Har. That will change I promise you.

 

[Treebore]

Book of the Righteous and Alchemy and Herbalism and Oathbound and Dungeonworld. All very plunderable and useful in so many ways, far more than i expected. Especially Book of the Righteous. Toolbox as well, never thought i would actually use it, let alone often.