Buying a grammar book is a time-consuming business and buying it when one is in a hurry leads to disappointment. Alas, this was what happened to me. I only had 15 mins in the shop and browsed through the selection quickly. I decided on the Essential Spanish Grammar because it was fairly cheap and I didn't think I was ready for the more comprehensive editions, as I am at the beginner stage. This book is really for the reader to work through chapter by chapter, each one having a theme. This is a good plan if that's what you want. But if you want to look up a specific grammatical topic, then it is less useful. The grammatical index at the back is nowhere near comprehensive enough. For instance, I wanted to look up how to use the definite article with days and dates. There was no entry for "date" or "days", nor could I find what I wanted under Definite Articles. However, when you do find the topic in the book, it is explained quite well. It is undeniably useful if you want to find out grammar and vocabulay surrounding a particular theme such as "possibility, capacity, permission". Each chapter isn't very long, so you won't be overburden with facts.
I note that the author also wrote the Teach Yourself Spanish course. So, I'm wondering if this grammar book is designed to accompany the course.
So, buy this if you want to work through a grammar book; and you will learn a lot of grammar. But don't buy this if you want a comprehensive reference manual.