Okay, this book has positive and negative aspects to it.
First the negative aspects. There are typos, which should be expected for such a huge book's first edition. Some are severe, others are not (e.g. when showing a rank-2 tensor is symmetric, it writes $X_{ab}=X_{ab}$ instead of $X_{ab}=X_{ba}$). These are the only negatives that come to mind.
The positives are that this is probably the most (if not *the*) authoritative reference on constraints in general relativity. It covers York's conformally formulated constraints, and Moncrief's contributions as well. This is done in a scholarly manner, so one can refer to the original sources, and in a self-contained cohesive manner (so you don't have to refer to the original sources). It is really quite beautifully written.
It may be a bit intimidating for the uninitiated working with constraints. I cannot help but apologize to the neophytes, I know no good introduction to the canonical formulation of gravity (except perhaps Poisson's book "The Relativist's Toolkit"). I cannot really deem this either a good or bad introduction, I was using the book as a reference.
There are exercises, however, so if the reader performs all the exercises...she will be far more astute when it comes to the computations and theory behind esoteric topics in general relativity.
But wait, there's more! This book covers black holes (in a rather mathematical way, so all of your favorite singularity theorems are here presented with proofs in a cohesive and beautifully well written manner).
My two cents is to refer to this book when working on canonical formulations of gravity; I am currently in a reading group on black hole thermodynamics, and this book is my first recommendation to other students in the group.