As I recently posted, I'm using O'Reilly's Safari online library service. So far I've really enjoyed the easy access to high quality documentation, and am very impressed with the overall service. I've been slightly disappointed with a few of the books I chose though, for instance:
The problem with the latter two items is that they take up two slots each! Ouch.
The ones I've really liked are:
Safari is not only a great reference tool, its also a really good "preview" tool to check out potential books to buy. I've made the mistake of buying a book thinking it is one thing when it is really about another, or that it is too advanced or basic for me.