what is a good link or book to learn group policy

this exam stresses on group policy and gpo
where should i go to get a good grip on group policy.
here are the books i have.
m$ book for 210, 215, 216, 217, 218
sybex for 210, 215, 216, 217, 240
exam cram for 210, 215, 216, 217, 220, 221,
mike myers passport 210, 215, 218
new raiders 240
osborns 240

please tell me which book helped you or if there is a website that helped you.
thanks