You'd think there wouldn't be 200 pages worth of material to writing a query letter, but you'd be wrong. This book is straightforward, practical, and on topic all the way through. It's more focused on articles and nonfiction, I'd say, so perhaps it's not the best book for fiction queries; but for nonfiction, it's pretty stellar.
What was particularly useful to me were the numerous examples, both good and bad. It's clear that the author has query letters down to a science, and she's not just pushing a simple recipe.
Not only is it helping me with my nonfiction book query, but I kinda want to give query letters for articles a shot, now, too.