Difford's Guide For Discerning Drinkers is one of the largest and most visited drinks-related websites. It has a large collection of cocktail recipes but more than that, it has a lot of articles on tools, techniques and ingredients.
Wikipedia's List of IBA official cocktails is a useful single page list of the classic, contemporary and new era cocktails selected by the International Bartenders Association (IBA) for use in their annual World Cocktail Competition.
The New Cocktail Hour: The Essential Guide to Hand-Crafted Drinks
Tenaya and André Darlington
This is the book that got us started. It presents the recipes roughly in chronological order and gives a little background and history for each one. It contains plenty of sections discussing tools, techniques and ingredients too. I particularly like the fact that they give specific brands for the ingredients in their recipes rather than leaving you to guess. We don't always agree with the quantities in their recipes, and always adjust them somewhat to our taste, but this book makes a great start.
The Essential Cocktail: The Art of Mixing Perfect Drinks
Dale DeGroff has earned a reputation for being the mixologist who almost single-handedly brought about the recent cocktail revival. This is a reduced version of his book, "Craft of the Cocktail," more suited for the home enthusiast. I like it because it is organized according to cocktail style. It offers a little of the history behind each cocktail and frequently includes modern variations alongside the classic recipes. He also offers tips based on his many years of bartending experience. I don't think I shall ever flame an orange peel but this is where I learned the best method of salting a margarita glass.
The Fine Art of Mixing Drinks
David A. Embury
This classic guide to the cocktail was first published in 1948. The author was a lawyer specializing in income tax and corporate law but he had a passion for mixing cocktails. He was the first to analyze the structure of a cocktail separating them into two basic types: the aromatic and the sour. He separates the ingredients into three basic categories: the base, modifying agents, and special flavorings and coloring agents. His chapter on "Roll your Own" illustrates how all of the recipes are related and brings clarity and structure to a subject that otherwise is just a random pile of recipes with funny names. No one is going to follow his recommended ratios any more but the book is still a delight to read. Long unavailable it has recently been reprinted.
Liquid Intelligence: The Art and Science of the Perfect Cocktail
You may not want to chill your drinks with liquid nitrogen or heat them with a red-hot poker. Perhaps you are not interested in the rapid vacuum-infusion of liquors, vacuum distillation or clarifying your fruit juice in a centrifuge. You may not even want to carbonate your drinks yourself with nitrous oxide gas. Even if you are not interested in the equation for estimating dilution ratio from alcohol concentration, you still want this book. Just for the discussion of ice alone this book is worth it. The details of the physics involved in the simple act of shaking a cocktail with ice are eye-opening. This book is a delight and bears testimony to the near obsessive depths of scientific endeavor possible when a true artist is in pursuit of a really nice drink.