Catalogues from Track of the Wolf and The Rifle Shoppe (online; the printed ones are out of stock, I think) will show you the wide range of locks available today. There are few books dedicated to flintlocks themselves, and most have to do with their historical development in Europe. The Flintlock by Torsten Lenk is a very detailed book about the styles of flintlocks on very high-end guns through their development focusing on French and some Belgian guns.