Good books for download

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