Have all of them
Understanding Exposure is still, imo, the best photography book available if you're starting out. The heaps of info packed into it, explained perfectly, is priceless.
Photographer's Eye is extremely unique. Haven't found another book like it. It really helps you see the world as a photograph (jesus that sounds so pretentious it's not even funny). Places I'd visited before reading it with my camera and hadn't bothered taking shots, suddenly made me look at things differently. Not only amazing for a first time read, but if you're having a mental block it can really help.
Workshop is awesome, those guys *love* their long exposures so get ready to buy a 10 stop filter :P Some great practical ideas in there as well.