Basic:

1. Basic Digital Electronics - From National Instruments
2. Interactive tutorial on Digital Electronics.

Intermediate:

1. Basic electronics
2. http://www.electronics-tutorials.ws/

General Electronics

1. Practical Electronics for Inventors
2. Getting Started in Electronics
3. Encyclopedia of Electronic Components Volume 1
4. Encyclopedia of Electronic Components Volume 2

Digital Electronics

1. Digital Fundamentals By Floyd