A curated list of (legitimately) free CS text books (I've removed a few specific to learning Python):
A First Course in Electrical and Computer Engineering by Louis Scharf, Colorado State
Artificial Intelligence: Foundations of Computational Agents by David Poole and Alan Mackworth, University of British Columbia
Bits, Signals, and Packets: An Introduction to Digital Communications and Networks by Hari Balakrishnan, Christopher Terman, and George Verghese, MIT
Code Like a Pythonista: Idiomatic Python, by David Goodger
Computational Geometry by Nicholas M. Patrikalakis, Takashi Maekawa, MIT
Foundations of Computer Science by Al Aho (Columbia) and Jeff Ullman (Stanford)
High Performance Computing by Charles Severance, University of Michigan
How to Think Like a Computer Scientist: Learning with Python 3 by by Peter Wentworth, Jeffrey Elkner, Allen B. Downey, and Chris Meyers.
Information Technology and the Networked Economy by Patrick McKeown, University of Georgia
Information Technology for Management by Henry Lucas, NYU
Information Theory, Inference, and Learning Algorithms by David MacKay, Cambridge
Philosophy of Computer Science by William J. Rapaport, University at Buffalo, The State University of New York
Principles of Computer System Design: An Introduction (Part II) by Jerome Saltzer and M. Frans Kaashoek.
Principles of Programming Languages by Grad Students, Johns Hopkins
Programming Languages: Application and Interpretation by Dr. Shriram Krishnamurthi, Brown University.
Prolog and Natural-Language Analysis by Fernando C. N. Pereira and Stuart M. Shieber, U Penn & Harvard
Python for Informatics: Exploring Information by Charles Severance, University of Michigan
Structure and Interpretation of Computer Programs by Jerry Sussman & Julie Sussman
via +Adam Liss
(who also alerts us to this collection of online courses:http://www.openculture.com/freeonlinecourses