The layout of this book is similar to Nunn's Understanding Chess Endgames. After a couple of short introductory chapters, the meat is 100 fundamental issues in middlegame play. Each of the 100 points receives only two pages of coverage. Because these are middlegames rather than endgames, each point typically only has two examples, rather than the 3 or 4 in the similar endgame book. So this is no in-depth training manual. However, Dr. Nunn has done his usual good job of chossing instructive examples with multiple points. Just as important, he invested a lot of time coming up with a list of 100 fundamental issues that were genuinely a good survey of the middlegame. They are divided into several large topics: Attacking play, certain material imbalances (for example, queen versus 3 minor pieces), themes in defense, pawn structures in the middlegame. He ends with the most common types of mistakes people make during the middlegame, including critical things such as automatically accepting sacrifices, over- or underestimating an attack, and overestimating the value of the two bishops.
It's worth listing the pawn structures he points to: Isolated pawn in general; isolated queen pawn; closed Ruy Lopez; Winawer French; Scheveningen; Najdorf; Caro-Kann (also the Slav); Sämisch King's Indian; Benoni. As you can see, this is hardly a comprehensive survey of middlegame pawn structures, but the 8 covered (not including isolated pawns in general) cover a terrific amount of chess knowledge and a large majority of games played. When you consider that other sections focus on pawn chains, hanging pawns, and doubled pawns, most important pawn structures are covered.
Similarly, the chapter on the attack nods to several classic sacrifices: Bxh7+ (obviously), sacrifices on H6, g7, and f7; and several standard sacrifices in the Sicilian, as well as looking at Rook lifts, the long diagonal, attacking a fianchettoed position, and more.
Overall, this book isn't a comprehensive training manual; such a work would probably be several thousand pages long. Rather, it's a good way to check your grasp of middlegame play. If you play through each section of the book, you'll quickly identify which areas of middlegame play you need to improve on. If you see nothing new in the section on the attack, but the sections on defensive play are a revelation, that's probably a good place to focus. Any unfamiliar theme is worthy of follow-up in your own study.
I'm just hoping that Nunn will follow this up with a couple more volumes on the middlegame, as he did with the endgame. There's no hint of this yet, but hey, I can dream, can't I?