I've been a fan of Cory Webb's website www.howtojoomla.com for years, and when his book came out I was sure that it would be well written, and I wasn't disappointed.

Even though this is titled as a book for "beginners" there are quite a few advanced topics covered in it.

Cory gives a good introduction to Joomla as a content management system and its theory of operation and a bit of the history of the project and then dives in to terminology and basic concepts as sites are built step by step.

Keep in mind the word "beginning" in the title.  There are more advanced Joomla books that you will want in your library as you go further with Joomla, but if you are trying to "wrap your head around Joomla" Cory wants to help you.