I think the Official Guide for the TOEFL ibt is the most useful book. First , because  I can learn a lot of useful phrases from this book. For example, I know “make yourself a schedule” which means you try to make yourself a plan about how you manage your daily time And also  “concentrate on” which means you try to focus on something . Second, I get to know something about US campus life. Because TOEFL ibt is based on students’ lives in US campus, so you can know how they study , behave and think. I am sure this is helpful for me because I plan to go there and study. In a word, this is the most useful book in my opinion .