There is little publish information about folders in particular in books. I do have a copy of "It's In The Bag" that I purchased from a bicycle museum in NY state a while ago. It is very thorough about folding bikes circa about 10 years ago. It is very informative. Some other books worth mentioning are ones that discuss military bikes or bikes used during wartime. Our civilian folders are actual desendants of these bikes. There is only one repair book that even mentions them.
The way I write here, maybe I should consider writing a book on these bikes!