If you're looking for computer books I highly recommedn O'reilly's safari.oreilly.com
It's a heck of a lot cheaper if you just need a chapter or two, or want to investigate a new language. Alternatives are going to Border's/B&K but the coffee is expensive enough to pay for a small safari bookshelf
One thing I've noted over the years of using safari though is that nothing replaces holding the book, so I usually end up only using Safari for reference.
I end up always going to either Amazon or Ebay for purchasing books online. I usually can be found in a local BooksaMillion or Barnes and Noble every week or two at some point.
I normally use Amazon unless it isnt available there. If not then I do not check any other specific places and just google around till I find what I need and buy from whoever shows up.