Good books for download
Warning, many links are broken. I'm not going to update this page, search for the books yourself...
Programming
Graphics Programming Black Book (Michael Abrash, 1997)
Very good book about optimizing assembly programming, VGA tweaking and 3D-programming.
Some of the contents of the book may be old and dated, but the way of thinking is not.
Algorithms and complexity (Herbert Wilf, 1994)
Game Theory Text (Thomas Ferguson)
Linear Programming (Thomas Ferguson)
Mathematics
Generatingfunctionology (Herbert Wilf, 1990)
Nice book about generating functions.
Puzzles
The Mathmagician and the Pied Puzzler