I scored 770 on the gmat (top 1%), so can give some advice:
For quantitative section, I recommend this book to both learn the concepts and practice some "hard style" problems before moving onto official GMAT questions: https://www.amazon.co.uk/Total-GMAT-Math-Jeff-Sackmann/dp/1453772316
For Sentence correction, I recommend the Manhattan Prep book, very good to learn the actual principles of grammar/sentence structure etc. I never used the other two verbal books (reading comp. and critical reasoning) but I heard they are also excellent.
To improve your essay (AWA), this online service where it gets graded with detailed feedback is great: https://www.awaprofessor.com/. Howe
Overall though, my biggest piece of advice is really just do as much real GMAT questions as possible – get the official guide + the verbal/quant add ons, and also download the real GMAT simulator. First 2 ones are free, then you can buy additional tests to do. Find that a lot of 3rd party sources online are not representative of real GMAT questions – especially for verbal reasoning, where I wouldn't trust most of the 3rd party material.
Finally, Gmat club is an amazing forum to get study tips, learn new approaches (e.g. how to approach the essay) and to also see explanations for questions you don’t understand.