Here is a shameless plug for my own blog, [Math ∩ Programming](http://jeremykun.com/). The difficulty ranges in how much mathematical background you need, but there are also some primers aimed at programmers, starting from this [essay](http://jeremykun.com/2013/02/08/why-there-is-no-hitchhikers-...).

I'd really love feedback from you on what you found approachable and what you found unapproachable.