Introduction to Digital Filters With Audio Applications (free book)

[nickb]

http://ccrma.stanford.edu/~jos/filters/

[jamesjyu]

I took several courses at Stanford with Julius Smith (the author of this ebook). He's really sharp, and explains complicated audio signal processing concepts very well. If you're even remotely interested in pursuing research in audio technologies, I would highly recommend the CCRMA program at stanford.

http://ccrma.stanford.edu/

[nertzy]

This was frequently useful when a friend and I made a VST instrument. It's now freely available at http://sohasound.com

[thomasswift]

thanks for the link