It's amazing what you can find on the web. I was looking for more information on Fourier transforms and stumbled onto an excellent resource via a series of lectures posted on YouTube through a Stanford University outreach program. Excellent course, and the home page has all the course notes, lecture notes and exams.
If you want a textbook for a certain course but don't plan to take the course just yet, used book stores can be a gold mine and so can ebay. Find out what edition the local college or university is teaching from then get the previous edition. The main changes from one edition to the next are usually the problem sets and the graphics, neither of which affect the material that is taught. There's little demand for out-of-date textbooks but they're perfectly fine to study or as references and can be had for a song. My best standby is still Google and/or Wikipedia. They provide the quickest answers for me when I need them.