I'm glad that this post led to the group of you all that took the opportunity to donate! Didn't think it would get that much traction, so it's especially nice to see some good come from it.
"Understanding Analysis" by Stephen Abbott is a great undergraduate level math textbook. The focus is real analysis (a rigorous treatment of single variable calculus). It's an excellently written text which will bridge the knowledge gap an engineer may have. After Abbott you can pick up and read books like Spivak where the beauty of calculus (from an engineering viewpoint) can really be seen. Both of these books have exercises, but they aren't tediously calculating derivatives. The exercises are fun and often mind bending problems you can spend hours thinking about, and really come out having learned something once you get the idea.
Are you referring to a channel? Or the client itself? I believe the channel startups on freenode has catered to the HN community (and was where I heard about it initially), however I'm not sure how many users it has online typically.
Dummitt and Foote suggested above is a great text on the subject and probably the best when it comes to a complete treatment of the theory.
The only complaint one could make is that it's written for mathematicians.
A more appropriate book for "lower level" students (in terms of mathematical maturity) would be John B. Fraleigh's "A First Course In Abstract Algebra".
