Any book on "discrete mathematics" ...

That is exactly what I was going to say!

Discrete math is not only absolutely essential, but also fairly easy to pick up. I can refer you to the book I used in school, though I have no idea how it compares to others: Discrete Mathematics, by Johnsonbaugh.

I would also suggest checking out the plethora of "journey through mathematics" type of books that come out regularly. My favorites — oldies but goodies — are the ones by A. K. Dewdney, particularly The Turing Omnibus. That one in particular is really about computation and algorithmics, but very fascinating, and closely related to discrete math, most of the time.