I found this Book quite helpful, but it is not free:
GarageBand '11 Power!: The Comprehensive Recording and Podcasting Guide byTodd M. Howard (May 1, 2011)
The "Missing Manual" series is even better, but there us no recent GarageBand version, afaik.
You can check out my new book "GarageBand 11 - How it Works". My books are released in the "Graphically Enhanced Manuals" series and as the name implies, I use a lot of diagrams, screenshots and rich graphics to explain the application. This gives you a better understanding than just reading through hundreds of pages of dry text.
Here is an example that compares the Track Area as "The Band" and the Timeline Are as "The Music":
You can find more screenshots of my book on my website.