Electronic theory
Downloadable volume with information about basic concepts in electricity, circuits, components and many diagrams

Basic electronics
An online book with simulations

Circuit Sage
A complete source of information to help you design great circuits fast.

Alex's Electronic resource library
An Online Guide to Useful Electrical and Electronic Information

Magnetic properties of materials
An introduction for the designer of electrical wound components to the part played by materials within the magnetic field, and a summary of the related terminology.