If you need books, I recommend:
1. Manhattan GMAT SC
2. PowerScore GMAT Critical Reasoning
I'm not sure why everybody recommends the LSAT critical reasoning
book vs the GMAT one. They're very similar. But I think you'll get thrown off if you read the LSAT one because the LSAT tests these reasoning 'games' which aren't on the GMAT.
Above all else, I don't recommend a book at all. I recommend doing the SC1000 and CR1000. Those will get you leaps and bounds ahead.