Linear algebra -- matrices etc. -- is the bit of my undergrad. mathematics that I use most often in programming.

I really liked the informal text by Gilbert Strang, "Introduction to Linear Algebra". The 3rd edition is quite expensive, but Strang's video lectures are available here:

http://web.mit.edu/18.06/www/Video/video-fall-99.html

For "getting" mathematics as a way of problem-solving, I suggest Polya's books, starting with "How to Solve It".