CS Principles: Big Ideas in Programming

by Mark Guzdial and Barbara Ericson, remixed by Jeffrey Elkner
Hands-on introductory computer programming using Python 3.

How to Think Like a Computer Scientist: Learning with Python

by Peter Wentworth, Jeffrey Elkner, Allen Downey, and Chris Meyers
An introductory computer science text book using Python.

Lessons in Electric Circuits

by Tony R. Kuphaldt
A free series of textbooks on the subjects of electricity and electronics