I like quality when I buy books. Hard-bound, glossy paper, nice art. So the books I like the most are:
FRCS
Call of Cthulhu (d20)
Manual of the Planes
Psionics Handbook
(ELH hasn't reached Sweden yet)
The only softcover books I like are:
Magic of Faerûn, and Lords of Darkness. (Don't have Silver Marches yet)
Unfortunately, the D&Dg, and F&P didn't do too much for me, would have prefered more info about the churches and organizations.
The Core rule-books are good too, but that goes without saying.
And most (not all) of my D20 non-WotC purchases have been disappointments, unfortunately. Both in in-game usefulness, and in quality. Sure there are some inspiring thoughts there, but somehow...