It really depends a lot on what level you are at and what type of knowledge you are looking to gain (general OS X guru advice/tips, pro networking or server info, creative applications like Final Cut and Motion, trouble-shooting for software or for hardware, Unix on the Mac, etc).
Here’s some general recommendations:
The Missing Manual – Tiger Edition (a must have for all OS X users): https://safari.peachpit.com/JVXSL.asp?x=1&mode=section&sortKey=rank&sortOrder=desc&view=section&xmlid=0596009410&k=20&g=&srchText=missing+manual+os+x&code=&h=0&m=&l=1&j=list&catid=&s=1&b=1&f=1&t=1&c=1&u=1&r=&o=1&n=1&d=1&p=1&a=0&page=0
Any of the Apple Training Series books (Most of them are great. Choose the subject that interests you most): https://safari.peachpit.com/JVXSL.asp?x=1&mode=list&sortKey=rank&sortOrder=desc&view=section&xmlid=&k=20&g=&srchText=apple+training+series&code=&h=0&m=&l=1&j=list&catid=&s=1&b=1&f=1&t=1&c=1&u=1&r=&o=1&n=1&d=1&p=1&a=0&searchview=books
If you’re a UNIX geek (or want to be), checkout titles from O’Riely like “Mac OS X Tiger for Unix Geeks”: https://safari.peachpit.com/JVXSL.asp?x=1&mode=section&sortKey=rank&sortOrder=desc&view=section&xmlid=0596009127&k=20&g=&srchText=unix+mac&code=&h=0&m=&l=1&j=list&catid=&s=1&b=1&f=1&t=1&c=1&u=1&r=&o=1&n=1&d=1&p=1&a=0&page=0
That should get you started. If you want recommendations for more specific subjects or applications post again.
Tony!