If you are new to the GMAT, Kaplan Premier is a great book to start with - it provides a good intro to the test (explains the mechanics, basic priciples, gets you familar with the test structure, etc), has solid strategies that you can use regardless of your level, and finally, comes with 6 Adaptive tests. Think of it a starter camera/computer/car/etc - you don't want to start with anything expensive before you know what you are looking for actually. On the flip side, this book will last you for 2-4 weeks and most likely will only boost your score with 50-100 points. If you want a greater increase (and have 3 months to study, look below)

Mark Edwards is one of my favorite authors. "The Devil's Work" is full of many unforeseen twists and turns,. I highly recommend this book. It is difficult to put down.

This book really made you feel for the main characters and the twist toward the end was unexpected. A very enjoyable read and I am looking forward to the fourth book even though I am ready for the happy ending for Marco, Leilani, and Addy.