The Powerscore CR
Bible is excellent and, in my opinion, an essential resource in one's GMAT preparation. I also used the Powerscore RC
Bible. It's a more powerful tool than Manhattan RC
and LSAT passages are more challenging, but do you actually need an RC guide? If you're performing reasonably well for RC, then just practice with LSAT passages under the same time constraints as the GMAT RC problems.
LSAT LR questions are pretty good too, but remember walker's advice and switch back to GMAT questions prior to the test.