Lessons in Electric Circuits
ibiblio.org
ibiblio.org
https://www.edx.org/course/circuits-electronics-mitx-6-002x-...
There's a lot of content, but chapter 3 alone is really helpful.
It looks like any corrections and additions made on All About Circuits feed back to this source too, but AAC has a slightly more web-like layout and links to additional materials (forums, worksheets, video lessons, etc.).