is quite popular, so that's probably the best option for someone who wants to start with the basics. If you're still having some trouble, consult GMATClub's guide. The two sources just mentioned are the best available for prospective GMAT test takers. Also, read as much GMAT Fiction
Once you're ready to move on, purchase Manhattan's SC guide and Powerscore CR
Bible. These are fantastic books for the verbal section. You might have to read them more than once in order to fully absorb the material, though.