superphily - You would need more than the official guides to prepare for the GMAT. I would highly recommend that you enroll in a structured online course. To answer your question, yes, the 300 new questions available in the latest official guide is indeed very useful. Please remember that you must stick to official questions while practicing verbal. For quant, I would recommend
GMAT Club Test Center.
GMAT Club Test Center is the next best option after the official sources, including GMAT Prep and official guides. The questions available in the
GMAT Club Tests are fairly representative of the ones on actual GMAT. If you do not have access to the
GMAT Club Tests, please purchase the tests as early as possible.