you really need to search around for ebooks.. you probably will be finding pdf's instead of txt's.

search google for "oreilly ebooks"

this link has a few good things but they are pretty much higher level stuff.
maybe you can grab pieces that might apply.

http://www.comms.engg.susx.ac.uk/fft/

somewhere I had saved other links for ebooks I'll look around and pm you if I can find them.