mrfrantic
Any other study material that I can start with apart from this?
Once you've seen all the types of GMAT questions and gotten some practice answering them, you could take an official practice test from mba.com to see where you stand.
Armed with that information, you'll be in a better position to determine what types of materials would be optimal for your prep. For example, if you are close to your score goal, for example 30 points below it, then the OG may be all you need for achieving your goal. On the other hand, if you need to increase your score by 100 points or more, then it would likely be best to get some other materials that would teach you concepts and strategies and support your practicing a lot to achieve your goal.