I very much recomend the first two chapters of
Digital Audio Signal Processing: An Anthology, ed. by J. Strawn

these are

"An Introduction to the Mathematics of Digital Signal Processing" by F.R.
Moore

and

"An Introduction the Digital Filter Theory" by J.O.Smith


They require no more than basic algebra to begin, and Dick Moore's chapter
introduces
the analytic geometry and calculus it uses in perhaps the clearest and most
gentle way I have ever seen! Heck, I am a *trumpet* player, and I understood!
;-) Similarly, I think Julius' intro to filter
theory is a model of clarity. Three thumbs up!

Good luck!

CharlieB