Firstly, make sure that if you buy this book you are either really good at Cash games or you have read and own the first book in the series, Harrington on Cash Games 1.
This book focuses primarily on good turn and river play, as if you are playing good pre and post flop play against what stacks you have, then most of the bets on 4th and 5th street will be for a large proportion of your stack, so this book will help that critical large amount of your stack stay intact!
A definate read, with loads of hand analysis and interesting discussions on key moments of the hand, you get a great insight into how Harrington plays the game, and you certainly go on a journey that will definately improve your play.
Buy with the first book!