Treants are, perhaps, an unusual topic for a sourcebook. Despite the role of the ents in Lord of the Rings, they have never received much further coverage in fiction or fantasy RPGs. This book redresses the balance with a thorough look at the race, their psychology, and their magic.
Although character classes, feats, and spells are provided for treants, their distinctly inhuman nature does, to my mind, make them most suitable as NPCs. However, a GM wishing to use them as such will find much here of use, and its well written and evocative.
The treants of the MM are NG, but this book provides plenty of alternatives that make them more useful as a foe in traditional games. There are 'shadowed' treants here, that take vengeance against axe-wielding humans to an extreme, but also a couple of varieties of undead treant (both of which make perfect sense, and don't just feel like something tacked on for the sake of it), as well as three other evil or monstrous creatures based on the treant theme.
All in all, this provides rather more useful material than might be expected from its coverage of an obscure and good-aligned, but not very PC-friendly, race might suggest. Its well written, has good artwork, and gives a real opportunity to have a neglected race from one of the founding works of fantasy literature have a key role in your campaign.