There is no one "best" grammar book. Depending on your needs and level, different books may be more suitable for you. For example, if you are a native English speaker who wants to brush up on your grammar, a simpler book like "Grammar for Dummies" might be a good choice. However, if you are learning English as a second language, you might need a more comprehensive book that covers all the basics, like "English Grammar in Use." There are also many specialized grammar books that focus on specific topics, like grammar for writers or grammar for business English. The best way to find a grammar book that is right for you is to look at reviews and ask for recommendations from other language learners.