No doubt books are very useful learning tools. However, depending on the code, source code can be equally if not more useful. For example, years ago I learned a lot about C by reading (and later writing) Linux kernel source code.

It's also true that code being open source doesn't mean the code is useful, but a book being published doesn't mean the material is presented well, either.

Maybe it's just my pro-open source bias, but it feels like you've got an axe to grind against open source. If so, would you mind sharing why (maybe you know something I don't)?

Documentation is great in that it can elaborate in much more detail, especially with actual pictures and graphs.

However, unlike source code, documentation cannot be machine tested for correctness.

Honestly, they're both important, and for different reasons. In this instance, Cydrak and I were able to pass revisions of the ARM core back and forth to fully debug it.