Books and references. A good reference library can be costly, but you can start with a few basics with just a small investment. Your first book should be the Complete Price Guide to Watches 2016 by Richard E. Gilbert, Tom Engle, and Cooksey Shugart. The 1200-plus pages provide valuable information about watch movements, as well as tips for identifying pocket and wristwatches. The Rolex Report by John Brozek is a little out of date, but extremely useful for understanding Rolex watch varieties, parts, and detecting fakes. The Best of Time Rolex Wristwatches: An Unauthorized History by James M. Dowling and Jeffrey P. Hess is slightly more recent and also useful in understanding this highly popular brand. A subscription to Wristwatch Annual provides specifications and pricing of current models released by each major watch brand.
If appraising collectible watches is your specialty or, if you have money to burn, I suggest you make your way over to mondanibooks.com, which publishes some of the most comprehensive watch references. It took me a decade to save, but I am finally the proud owner of eight volumes of Patek Philippe and Rolex watch reference books. The books provide detailed information on movements, dials, and case backs of all models produced by these two brands.
NAWCC library. Membership in the NAWCC allows you access to the most extensive watch references, catalogues, and books in the country, many of which are out of print. If you have a research project, the librarians can be extremely helpful.