This book has been a classic reference on English grammar, structure and usage ever since the 1983 first edition. It is comprehensive, thorough, and clear. The organization of the topics is a model for teaching. For my own ESL teaching I frequently look here for detailed explanations and examples which can be found nowhere else. The second edition has updated the grammar treatment according to the latest theory, is even more methodical than the first edition, and introduces valuable new topics.
Now pay attention, because you might not want this book at all! It's a technical reference book for English teachers with good prior training. Before you spend a lot of money on it, make sure it's what you want! Don't expect it to be something it has no intention of being. It's not a textbook for learners, and it's not for casual readers, beginners, or ESL students. People with no training in linguistics or grammar will find it above their level, so be warned! There are many more useful and cheaper books for you out there.
If you're an English teacher or planning to be one, if you're really interested in the "why" of grammar and usage, and if you don't mind detailed technical explanations, then you will find "The Grammar Book" invaluable. It adds to your understanding of every topic, offers fascinating insights into the workings of language, and has really useful examples everywhere.