The only two high on my wish list are Ultimate Adversaries and Coruscant and the Core Worlds, both of which I have found multiple copies of in local game stores. When I get the cash, they will be mine. I already have Hero's Guide, Geonosis and the Outer Rim, Galactic Campaign Guide, Ultimate Alien Anthology, and Arms and Equipment.
The other books are not "must haves" for me, compounded by the fact that they are all so far out of print that I know I'd end up paying beau coup bucks for them on eBay. I know where I can get a Secrets of Naboo, too, but I'd really rather have Secrets of Tatooine -- that planet is as much of an icon as any of the main characters.