Buying text books for university is pointless! I just wouldn't bother, there isn't a text book that you can’t find online for free, most lecturers will tell you not to even bother with the books, they are added in to the course so that the course gets past the examination bored, bunch of middle managers.
Anyway, lets do this thing;
Your first stop for any question, Google indexes a massive part of the web and using Google advanced search operators you can find just about any book, also buy just typing in the ISBN or book title can also find good results.
Google book search is Google’s way of bringing all literature to the world for free, with over a million titles it will more than likely have the book you need from classics to magazines. http://books.google.com/
Is a good online resource for finding them allusive PDF files, most are useless and I wouldn’t bother downloading but there is the odd gem and the site shouldn’t be over looked. http://www.ebooknetworking.net/
Is a gem of a site that I wouldn’t have got my A levels without, “over a million titles” if you are on a computer course or if you need to use a computer for your course the you probably can't do without this massive archive.
Wikibooks is a collection of open-content textbooks that anyone, including you, can edit, like its big brother Wikipedia it’s massive and a good place to start any project. http://wikibooks.org/
I was going to save this for another post but when you do your thesis you are going to need Google Scholar, it indexes all of the known scientific papers and can be extremely useful. http://scholar.google.co.uk/