Does anybody know of any useful, accessible, and manageable (preferably free) online textbooks for undergraduate math courses? I found this one (http://www.math-cs.gordon.edu/~kcrisman/mat338/index.html) for Number Theory very good when I used it and I'm wondering if anybody knows of any others.

I think a bit of time googling this question might have led somewhere, but perhaps not. One professional mathematician made this suggestion:

Readers of this blog (if any) will know about the following. But I repeat it here in case there is anyone out there asking the same question and stumbling, for the first time on this blog.11 hours ago

I believe that this is the future of textbooks, especially in mathematics. Having published with a major commercial publisher of textbooks who shall remain nameless (Pearson!) who priced our textbooks in the stratosphere, I find now it much more sensible to offer free PDFs and inexpensive paperbacks directly.Students need all the financial assistance they can get nowadays. Free textbooks are an excellent way to help in a small way. We academic mathematicians can make better use of our expository talents than enriching the commercial publishing industry.

All of our textbooks are online at

http://classicalrealanalysis.info/com/Home.php

and PDF copies are free. Mostly these are real analysis textbooks from an elementary level up to graduate.

Also there is a more general source of free mathematical textbooks available

at

http://aimath.org/textbooks/approved-textbooks/

collected and authenticated by the American Institute of Mathematics.

http://diestel-graph-theory.com/