I just found some awesome free books to download in PDF format:

• Security Engineering: A Guide to Building Dependable Distributed Systems (1st Edition) by Ross Anderson
• Information Theory, Inference, and Learning Algorithms by David MacKay
• Programming the Be Operating System, by Dan Parks Sydow
• Numerical Recipes in C & Fortran
• Introducción a la programación con Python e Introducción a la programación con C de Andrés Marzal e Isabel Gracia (in Spanish)

Happy reading!