What is the best book to prepare for this CLEP exam other than Princeton Cracking...? Is it a hard exam or the easiest one?
It's easy enough that some schools won't accept it for college level English credit. At which school are you planning to use it?
I used this one - http://www.amazon.com/exec/obidos/tg/detail/-/156030183X/qid=1071203023//ref=sr_8_xs_ap_i0_xgl14/104-0224834-7979139?v=glance&s=books&n=507846 Review for the CLEP General English Composition Examination - ISBN 156030183X
This test isn't that hard - Princeton's Cracking the CLEP is by far the best (and eaiest to read) review on the subject, IMHO.
There are two seperate tests; As Jerry mentioned, some colleges (my undergrad included) won't accept the CLEP English Composition w/ Essay as satisfying their GE requirement. Many more will accept the CLEP Freshman College Composition w/Essay. I've taken both, and there is not that much difference between the two, other than the Freshman Comp seems to focus a lot more on syntax and sentence structure. Even then, some schools won't accept any CLEP or DANTES exams when it comes to English courses