If I were to pick one book to study from. it would be the OG. The fact that the problems are more difficult is a GOOD thing == especially since the OG provides complete explanations for every question and answer both right and wrong.
If you really want to learn this stuff, go through the OG and do not leave a question until you are completely able to understand and explain in your own word both correct and incorrect answers.
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993