Take it from a professional sabermetrician: this might be the best and most important single volume in our field. It's a splendid complement to Baseball Between the Numbers, addressing many of the same questions but in many cases digging deeper. The authors have impeccable reputations in the online sabermetrics community. While they can't match Bill James for wit or BP for snarkiness, the writing is clear and solid.
I don't think that any of the findings here (some of which are truly eye-opening) will end up as the very last word on the subject: but more often than not they are the latest word. And that makes the book an essential purchase for anyone serious about understanding the game of baseball.