Regan's book is pretty much essential, and DeGroff's is great as well. Most
"classic" cocktail books, I find, aren't really geared toward modern-day
bartenders so much as cocktail enthusiasts and historians. They're fun to
read, but not as real-world applicable as the more current books.
Mr. Boston's guide, as much as it gets maligned, is also a handy thing to have
around if you quickly need a drink recipe that you're not familiar with. The
latest version is much improved over the earlier editions.