Of course I have to start by saying you don't need any books if you try our program, Economist
GMAT Tutor. In fact, you don't even have to take notes!
Although our course is a fully comprehensive one, if you feel you need more practice, the only resource we would recommend is the Official Guide for GMAT Review (the 12th and, especially 13th edition), which is a resource of real (retired) GMAT questions, so you can add a bit more time to the above hours. Please note that generally you should only use this book once you have completed most of our program as it is only a practice book and does NOT teach a student how to attack each question type.
Also note that we already have 5000+ questions in our program so the Official Guide, which only has about 850 questions, contains fewer (but good) questions. Also note that the Official Guide does not always provide the most proper explanations, so do rely on our lessons and techniques even if you use the OG (this is also why I strongly suggest doing the OG a bit later in our course).
The point is: The Official Guide is an excellent extra resource. Use it in conjunction with our program, not as your main study resource.
Economist GMAT Tutor