Via a comment in the proggit thread about an online Haskell and music theory book mentioning Guerino Mazzola's very intimidating The Topos of Music, I ended up looking up the author and finding that he's also written Comprehensive Mathematics for Computer Scientists 1 and 2. These look intriguingly compact, while staying at a sufficiently fundamental level that someone with a modest theoretical background might still have a chance of following them.

Now I'll just need to try to actually read them properly.