A good thing that we used to subscribe to was O'reilly's safari - where they publish all of their books online in a searchable format. You pay a subscription each month and can have a number of books on your "bookshelf" at one time. They give you a free 14 day trial and it could be worth a look as you get the whole books online and if you prefer reading proper books you can always buy the book if it's useful.
http://safari.oreilly.com/