I had 2 months of study time while working.
I studied 1 hour in the evening during the week and 4 hours on Saturdays and Sundays.
I had all kinds of GMAT books, but the one which really improved my score was:
Kaplan GMAT 800
Advanced Prep for Advanced Students.
I know Kaplan
isn't really a favourite here. But that book gives you practise for all the hard concepts. After I finished that one, my math and verbal score increased dramatically in the practise tests. Especially in the Sentence correction. The book is for advanced people who know the concepts of the GMAT and it only targets the difficult questions, so you don't waste you time doing simple questions which you anyway would get right.
Therefore I can recomment to use that book, once you have a solid foundation about the GMAT and its concepts.