Sorry I have to plug safari, I found it to be a truly great service and is changing my way of approaching tech books.

I have it; I found it very useful. The great thing is that you can check out a book without buying it. This avoids the "shelf-creep" that happens when you wind up getting marginally useful books that you hate to throw out, but never really use. They just take up space. With Safari you can read the entire book if you want, and decide whether it's worth adding to your paper library. It has already saved me a pile of money on books, as well as allowed me to check out subjects that I would otherwise have skipped because of the price of buying a whole book on the subject.