Hi Kenneth,
It is good you already have a a general idea and basis on what you need to be focusing on to improve your score. I would recommend starting with Verbal. Before really diving into the practice questions, I would go back and study the basics of grammar. Do not move on until you have them mastered. If you are tackling the regular GMAT problems and are missing a few of the key yet lesser known concepts of basic grammar, it is enough to tear you down when taking the GMAT. Once you're comfortable and you have moved on I would continue studying both verbal and quant at the same time. If you can, sign yourself up for a good adaptive course that will determine your weaknesses and teach you in an applicable manner. If you need any suggestions for any let me know. After you start to feel comfortable with the verbal, if you want a good supplemental book to advance your studies with, I would recommend "English Grammar in Use" (intermediate and advanced levels). Please be aware that almost all grammar books tend to be exercise-oriented and often go into much more detail than what the GMAT tests. In addition, many grammar books do not deal with certain sentence structure issues (e.g. dangling modifiers). You need to look for something that can help in that regard too, or simply fill that gap with a proper course. You need to cross-reference what is on the GMAT with the book you get. But "Grammar in Use" is probably the best grammar self-study book available. If you have any other questions please feel free to ask. Keep up the good work!
Sincerely,
Evan