Hi Jane, there is at least one invalid feeder collector group, and a Google Search "Invalid feeder" will turn them up. I would assume they could answer your question. Google search also turns up other sites which may be of use. Please let us know if you come up with a book, I'm sure others would be interested.
There used to be a terrific web site "Ceramic Infantilia of the 19th Century", but sadly that site is no longer active and the URL is for sale. Sad. It is conceivable I may have a contact for the former owner of that site, I'll try to locate it, but it's a long shot.