Just got a new book about Oracle from Amazon. The book Cost-Based Oracle Fundamentals written by Jonathan Lewis describes the Oracle Cost-Based optimizer in a easy and understandable way. Jonathan Lewis is the director of the UK Oracle user group and is the author of several quite good books, this is however his best book so far.
My only disappointment so far (haven't fine read all sections yet) is that the book doesn't really cover how to "manually" influence the CBO in other ways than SQL HINTS, such as "faking" your own DBMS_STATS and costs.
Any how, the best book on Oracle I've seen in a long time.
Go get it, both for DBAs and developers.