Discuss the aspects of the book that affected your life. If you're writing a testimonial about a book that helps people strengthen their faith as Christians, mention specific passages that stood out for you. For example, discuss how the author said that regular Bible study is important when facing strong temptations, and that you learned how to refer to certain Scriptures when you faced temptations in your life.
Mention reasons why people should buy this book. If you read a book about how young men can become more financially and emotionally mature in their lives, state that the book encourages young men to do this by increasing their savings, earning extra income if necessary, learning how to solve problems on their own, and building integrity in their careers and personal relationships.
Compare this book to others that the author has written. For example, if you're writing a testimonial about a book in which the author discussed the problem of childhood obesity, mention that this is a continuation of the author's previous book, which dealt with the negative effects of easy access to fast-food establishments in low-income neighborhoods.