The first step is to consider what you're looking for.
Are you looking to gain an understanding of the UMAT, along with different question styles and ways to solve problems systematically? Your answer is The UMAT Manual: A Systematic and Simplified Guide. It is the ONLY UMAT textbook created, aiming to provide a step-by-step method for the most common questions of the UMAT, with unique and optimum approaches to tackling these questions. This book outlines the reasoning behind the question, breaking down the UMAT into simple and bite-sized pieces. The name gives it away; it's easy to follow and extremely handy. Best of all, it comes with a full-length, hard-copy practise exam.
Are you finding the UMAT stressful, and looking to hear some varied, applicable tips from a wide range of high-scoring students? Look no further than The UMAT Files: Secrets of Top-Scorers. This features a wide range of high-scoring students and focuses on both the psychological and intellectual aspects of the UMAT, with personal tips and invaluable advice. Each author writes his/her own article, delving into personal experiences and lessons learnt, as well as exploring a wide range of preparation strategies. It also comes with a full-length hard-copy practise exam.
If you could only buy one book, it'd have to be The UMAT Manual: A Systematic and Simplified Guide. But if you're looking for a well-rounded boost to your score, check out The Ultimate Package!