I like O'Reilly's book "Mac OS X in a Nutshell" (I'm a Unix admin by profession) but my girlfriend prefers "The Missing Manual".
Both good books, I think, but each has a different focus. Try picking one of them up in a bookstore, imagine a question you might ask about moving to a new OS (how do I burn a CD?... how do I type extended characters... etc.) and see how easily you can find the answer in that book.
Depending on the question you ask and how helpful you find the text, you may find neither of those books is right for you. Or both are.
Good luck with your new Mac!