In case you didn’t know, Jonathan Lewis’es new Oracle Core: Essential Internals for DBAs and Developers book is out (for a few weeks already).
I was the technical reviewer for that book and I can say it’s awesome! It will likely be the best Oracle internals book out there for the coming 10 years, just like Steve Adams’es Oracle Internal Services book was in the last decade :)
Jonathan does a very good job explaining complex things in a simple enough way – and the book is not just dry listing of how things work inside Oracle database, but also why they work like they do and what are the benefits, limitations and side effects of the behavior.
So check out Jonathan’s book page for more details (and I think you can still order a heavily discounted ebook from Apress too):