The GMAT Official Guide (OG) differs from other GMAT prep books primarily in its source and authenticity, as it is published by the Graduate Management Admission Council (GMAC), the creators of the GMAT. It contains real past exam questions, making it the most accurate representation of the test in terms of difficulty, question types, and format. Other prep books, published by third-party providers such as Manhattan Prep, Kaplan, or Magoosh, offer strategies, practice questions, and explanations designed to supplement learning, often including additional tips, sectional strategies, and more extensive problem sets. While the Official Guide provides the most reliable practice for understanding the test’s true style and rigor, other prep books contribute by teaching effective approaches, reinforcing concepts, offering varied practice questions, and helping students develop time management and test-taking strategies. Using both in combination ensures comprehensive preparation, balancing authentic practice with skill-building techniques.