Recently cocktail writer Gary Regan rebranded himself as Gaz Regan and wrote a new book called The Bartender's Gin Compendium.
The history chapters are informative but not so in-depth that you'll get bored with them. The Old Tom chapter was particularly good and more information on the subject than I've seen elsewhere. There is also some great background about a few particular cocktails including the Tom Collins and Singapore Sling. But really, the focus of this book is practical knowledge about gin, and the title is appropriate in that it is really written for bartenders and other industry folks rather than for consumers.
I quite enjoyed the breakdown of the botanicals commonly used in gin- what they are, what they taste like, and what they do for a gin. We learn that some ingredients exist not for their own flavors, but to marry other ingredients in the recipe.
Regan largely let brands describe their own products, for better or worse. He gave each brand a chance to list the botanicals used their gin and explain why the gin is different from other brands. The thing I would have liked here is a mention of whether the list of ingredients was all-inclusive or a partial list. For example, Martin Miller's gin has always talked about their secret flavors that everyone recognizes as being cucumber, yet cucumber is not listed here as an ingredient. Thus it's a partial list.
Still, knowing key flavoring agents in different brands will be useful to the taster who is trying to analyze each gin, and to the bartender who wants to know what other ingredients to pair with it in cocktails. The important thing about gin is that it helps embrace and echo other flavors in cocktails. This book lists many, many great gin recipes in the final chapter, but I think with the information about what is in each brand its best utility is as an ingredient guide to help bartenders create new cocktails going forward.