The books i have for redhat/fedora are indeed like you say, centred on it's GUI, o'reily does some great cookbooks with various different tricks and commands, and they also have some sys admin books. To be honest if everything is done at the CLI then you can use almost any Linux book, the problem i find is alot of them were written <2000 and seem a tad out of date.
Personally i would try and look at past RHCT papers, and look at the sort of thigns it asks, then install redhat (fedora is free and mroe cutting edge) and have a go at basic things like adding users/checking logs/writing simple scripts etc. This may also impress your employer that you are taking a pro-active approach to this new position. In conjunction with learning on a system there are PLENTY of online resources to help you, and they tend to be mroe uptodate than printed literature.
- rute, a brilliant read and learning tool
- The linux documentation project - Lots of differnt articles and tutorials