Yes, the official guide is really useful for GMAT. The official guide and GMAT supplements are part of the part of the Graduate Management Admission Council, the owner and publisher of the GMAT. These books contain real GMAT questions, which can be used in actual GMAT examinations. This means these questions are exactly like those you can expect to see when you take the official test. In addition, because the questions are used by a large number of test takers, who are able to print the test questions in difficulty order. This means it can very efficient to understand, and must add all the answer and explanation written by professional test writers. This official guide also contains diagnostic tests, skills review, and test taking strategy and practice test questions.