You may have noticed a sudden appearance of Fourier transform stuff around here. It's one of the things I've been meaning to get around to playing with for a while, and when I saw Matt Jockers' Syuzhet stuff, I suddenly had a reason. Here's a couple of good links I've found so far:
The Scientist and Engineer's Guide to Digital Signal Processing, by Steven W. Smith, Ph.D. A hefty textbook on signal processing, convolution, Fourier transforms, and so forth, with "Very readable - low math - many examples". It's pretty easy to get bogged down in the math, especially if you're like me, and I know I am. This book looks at the topic from a discrete, programmable standpoint and a use, not derivation, standpoint. It's available online; I'd suggest the PDF versions of the chapters since the HTML version is missing for some chapters.