Textbooks are incredibly expensive (not to mention cumbersome and heavy in dead-tree format). Smart students can pick up free digital textbooks from online sources, including one we haven't mentioned before: Rice University's OpenStax College.
This nonprofit initiative is supported by philanthropic foundations and the peer-reviewed textbooks are provided to over 200 universities and colleges, as well as individual students. Currently, about a dozen textbooks are available, covering mostly the sciences, but history, economics and other subjects are coming soon.
The OpenStax books are available in PDF and EPUB formats, and you can also read them online (even from mobile devices). If you want, you could also customise the cover of the textbook or even rewrite sections of one and have it printed.
Although the selection is small right now, this is definitely a site to bookmark to keep an eye on for the next semester. Low-cost, open education FTW!