Useful Online Advanced Math Textbooks?

On LinkedIn just recently a mathematics student, Benjamin Bowden,  posted this question.

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:

There is Diestel's book on graph theory. Starts from the elementary but soon goes to the advanced.
http://diestel-graph-theory.com/

11 hours ago

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.

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.

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.

 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.