The official guide for GMAT review ( 10th edition)
This book contains about 1500 actual questions from previous year exams. It's the closest match with the actual gmat test. Specially in verbal section, where ETS spend a lot of money to prepare a single question. Questions choices are prepared very carefully to maintain the same level of difficulty throghout the years. In quantitative any body can change the numbers and can prepare the sample questions. So based on our experts experience we suggest one should practice verbal section only from official guide. For quantitative sections one may refer other books.Expert's TipIn quantitative section the questions are difficult in actual gmat. In the book initial questions of quantitative section are very simple but 200 questions in end are difficult, which closely matches the difficulty level of actual gmat questions.In comprehension section of verbal section one gets four passages in actual gmat test( Each passage has 3 to 4 questions ). In gmat book for some passages 6-7 questions are given.
Old actual practice test papers
If you find the above book not enough for practice then one can buy old sets of papers from http://www.mba.com/ site. Please note in these sets most of questions are new but some questions are repeated from official guide for gmat.
Kaplan GMAT 800 ( latest edition)
I would recommend this book for quantitative section. Here level of difficulty is more and it prepares u for more rigrous gmat than the actual so one ends up doing well in the exam.
Power prep test series
These are available free of cost and they simulate the actual test.
Free GMAT test prepration software 2006 Expert's Tip
Please note they also carry some questions from official guide so you may score more as you have already practiced some question from Official guide for GMAT.
Other sources ( kaplan's and Princeton's regular classes )
Based on our experts and their students experience( all who scored around 750) feel that above sources are enough but incase you are still not scoring well in the exam then one can join the regular classes by Kaplan or Princeton review. They give u regular practice and costs around 1000 USD.