Hi Km2014,
The recent main Official Guides (GMAT2015, GMAT2016 and GMAT2017) are all similar books. From one to the next, a certain percentage of questions are 'swapped out' (10% to 25% from version to version), but any of those books would be fine to use during your studies (and you could even use one of the older books and still be fine). That having been said, while the OGs are great books for practice questions, they're not designed to teach you the Tactics, patterns or the little 'secrets' of the GMAT. For all of that, you would likely find it easiest to invest in a GMAT Course of some type (either Guided Self-Study or instructor-led).
Before I can offer you any additional advice, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied?
2) What materials have you used?
3) How have you scored on each of your CATs (including the Quant and Verbal Scaled Scores)?
Goals:
4) What is your goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School?
7) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich