The Antique Doorknob Collectors of America.
Here is the link to their website: www.antiquedoorknobs.org/
They have a dandy web site, which will give you an overview of doorknob styles and classifications. They also have a small hardware-related on-line bookshop where you can obtain some wonderful resource literature. Sometimes your local public library may have one of the publications noted so you can continue your research there.
One question inevitably pops up—one that’s seemingly innocent enough: what are my knobs worth? Unlike cars or coins, there are no set “book values” for old doorknobs or hardware. The number of collectors versus the numbers of objects is a case of too few collectors for far too many knobs. (Think about it: how many doorknob collectors do you know personally?) Even Web Wilson’s great hardware book “Antique Hardware Price Guide” will only hazard a rough price range for the objects depicted. If you wish to sell to dealers or collectors, there is necessarily a wide spread between “wholesale” and “retail” prices. Frequently (dare I say usually?) doorknob collectors will simply trade hardware amongst each other, with dollar values not even entering into the active conversation. This isn’t to say your great set of old doorknobs is “worthless”—-but that others in the field recognize that there is a value there that isn’t merely monetary.