One of the BeagleBone Black's strong suits is the large community surrounding it.
The official site at http://beagleboard.org/ has lots of great information.
The main source for help with BeagleBone-related issues is the mailing list at https://groups.google.com/forum/#!forum/beagleboard.
There are also plenty of helpful people on the #beagle IRC channel at http://beagleboard.org/Community/Live%20Chat.
There are also many resources online that can help fill the gaps this book leaves on the electrical side. For instance, the Element14 community at http://www.element14.com/community/welcome and the EEVBlog at http://www.eevblog.com/, both contain a wealth of great material, as well as very active electronics forums.