The one sentence book summary is something that I learned from Ryan Holiday. His post on 42 books that will make you a better person used the technique to provide stratagems into some of the most important books he has ever read.
Using just one sentence to clearly convey the most important message of a book is a tough task. It is not always the thesis or the major point, sometimes it is a small detail that makes the biggest impact.
Either way, it required me to deeply reflect on each book from this past year.