We strongly recommend that you buy at least two books to prepare for the GMAT.
Firstly, you should get an official guide. The book will contain actual questions from prior GMAT exams. There is a significant difference between the official questions you will be asked on the exam and the practice questions contained in all other books. With the official guide you can familiarize yourself with the quality of questions you will encounter on the GMAT.
Secondly, you should consider buying at least one of the "unofficial" guides reviewed below. Knowing the timeproven testtaking strategies would help to improve your score. The better unofficial guides contain many such strategies, as well as explanations and timesaving techniques to answer questions.
You will greatly increase your chances of receiving a higher test score if you prepare yourself with GMAT testtaking strategies and knowledge of actual questions from previous exams.
The GMAT Official Guide 2018 Bundle consists of The GMAT Official Guide 2018, The GMAT Official Guide 2018 Verbal Review, and The GMAT Official Guide 2018 Quantitative Review. This bundle features actual questions from past GMAT exams  including 220 neverbeforeseen questions. It also come with exclusive access to gmat.wiley.com, where you can build your own practice sets using the questions in the guides and access an additional 58 integrated reasoning questions.  
Manhattan Prep's GMAT Strategy Guide Set is an excellent bundle of 10 guides: GMAT Roadmap, Number Properties GMAT Strategy Guide, Fractions, Decimals, & Percents GMAT Strategy Guide, Algebra GMAT Strategy Guide, Word Problems GMAT Strategy Guide, Geometry GMAT Strategy Guide, Critical Reasoning GMAT Strategy Guide, Reading Comprehension GMAT Strategy Guide, Sentence Correction GMAT Strategy Guide, and Integrated Reasoning & Essays GMAT Strategy Guide. Each book delves deeply into a single area of the exam, providing detailed and specialized instruction. When used together, these guides produce a substantial learning impact, helping students develop all the knowledge, skills, and strategic thinking necessary for success on the GMAT. It includes 6 full length computer adaptive practice tests.  
Cracking the GMAT includes a comprehensive subject coverage of all GMAT topics and exclusive free access to practice exams and further review online. Detailed answers and explanations are given for every question. Also has 6 fulllength CAT (comuteradaptive tests) practice exams online with score reports and detailed answer explanations. 

Kaplan's GMAT Complete 2018 comes complete with a comprehensive review of all the material on the exam, and testtaking strategies to help maximize your score.It also provides additional online features to help you practice with even more GMAT questions. There are more than 2000 practice questions complete with detailed explanations covering the Verbal, Quantitative, Analytical Writing, and Integrated Reasoning sections. 
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