Here's the cold hard truth. There are no good books for critical reasoning. If it's any solace, the CR for gmat is far easier than the LSAT.
Some people use the critical reasoning bible for GMAT. Take a look at the number of pages. It's huge!!!
Also, some people practice on the LSAT CR
questions to get used to higher level problems.
Neither of these situations presents a good answer to your problem.
My suggestion would be review simple Aristotelian(sp?) logic. You know, the if then clause, contrapositive etc. Do it briefly. Then get used to the questions format.
I bought some book that was useful (I forget the name of it, but check my debrief) it was simple but by far the best guide on CR.
Just remember that CR expertise comes mostly with practice. Remember that assumptions and conclusions are the most relevant part of the argument. Identify and maniuplate the assumptions and evidence to arrive at the conclusion that the questions asks for.