The documentation set can be a bit dry, and very voluminous, so if you want a book to ease you into it then try the 10g Complete Reference. [ December 11, 2006: Message edited by: Chris Hendy ]
I've read the book Chris suggested; in fact I wrote that review. If I was only buying one book, I would go with a book that just covered Oracle. Mainly because I like to try to avoid Oracle specific features. If you plan on writing stored procedures or otherwise tying yourself into Oracle, that is the book to choose though.
I haven't read The Complete Reference, so I can't compare either book to that.