I think the best overall guide for Grand Canyon river running is the Martin & Whitis guide put out by Rivermaps. It has lots of topo maps, and good information on camps, side hikes, points of interest, rapids etc. However, for geology, the Belknap Guide is a little better, with some more detailed observations.
Theres always these two competing philosophies about river maps as well, where some prefer not to know where the camps are and just want to discover things as they go, and others like to know where all of the camps are. If you're in the former catagory, then get Belknap guide, because its the only one that doesn't show camps, otherwise Martin & Whitis is the way to go.