MIT OpenCourseWare: Signals and Systems

http://www.dspguide.com/: a free but slightly buggy book on DSP by Steven W. Smith.

Audio-EQ-Cookbook.txt: a text file by Robert Bristow-Johnson. Also see this accompanying paper.

“Elements of Computer Music”, by F. Richard Moore, Prentice Hall

Introduction to Digital Filters; and in general Julius Smith III’s online publications.

Theory and Techniques of Electronic Music.

Wikipedia article on Electronic Filters.