Safari books online is ~$300/year but is worth it weight in gold. Most of the time you just need part of a book and need to look at several books to find the right answer or code snippet. Currently my company pays for it but I'm so used to it now, I would pay if they decide not to.
A lot of schools including my alma mater has this free for students/staff. They also allow alumni if you bought the lifetime alumni pack when you graduated which was $300 and included a bunch of other university services.
I have use the safari books extensively, but their book viewer sucks. I wished it was a much better UI.
