I can't claim to be much of an expert in cocktail making, being more interested in the consumption than the creation, but anything I have picked up comes from the books below.
Difford's Guide to Cocktails 7 by Simon Difford
This book is the definitive collection of modern cocktails, and most of the recipes featured on this site come from its pages. These guides started off life as a series of magazines, getting ever more comprehensive with each version. The seventh iteration contains over 2250 cocktails and comes in hardcover.
It has lots of useful features including an index by ingredient, which gave me the idea for this site, ratings, a section on bartending basics and the identification of "core" cocktails that can be made with a cocktail cabinet of 14 key ingredients.
However, the book has much more to recommend it, not least the sumptuous photography, with every cocktail getting a photograph. There is also a section on the history of cocktails and each major genus of cocktail gets a special feature on its origin and history. There are also drink related quotes from famous people peppered throughout from Homer Simpson to Bertrand Russell.
My only criticism is that some of my favourite drinks seem to have fallen by the wayside, which seems unfair when some pretty awful concoctions are given space.
The American Bar by Charles Schumann
This book is all about evoking a bygone age of cocktail drinking, of Hollywood glamour and hotel bars. All this comes from the traditional binding, the wonderful illustrations and the classic recipes. As the author says himself in the foreward:
"You will not find the usual thousand-and-one cocktail recipes in this book. Instead I have compiled five hundred diverse recipes that strike me as more than sufficient."
The author is an experienced barman and this is clear from the recipes. They are by and large classics that have stood the test of time and show off their ingredients in their best light. He holds that a cocktail should have three elements: the basis, the modifier and the flavouring agent which I think is a great tenet.