I am a switcher and liked "Mac OSX Missing Manual", lots of good basic info, even some UNIX stuff.
"Mac OSX for Unix Geeks" (O'Reilly) is not bad, a little thin in size and info, but a must if you are going to do UNIX SA/dev on your Mac.
I especially enjoy O'Reilly's macdevcenter.com, lots of detailed info on setting up Apache web serving, Oracle database, Samba, etc. on your Mac
Russell