That's an interesting project Allen!
Basically, the main source of information is the Higgins & Gage postal stationery catalog read in conjunction with Owen Ingle's chapter in "Tinsley".
That's about it I think except for the occasional journal article ... I'll have a look through David McNamee's bibliography and see if I can find anything that
may be useful.
I think that except that as a source of postmarks and postal history "interest" as well as privately overprinted notices, etc., the cards in their own right have been a little neglected by comparison
with the lettercards, wrappers, registered envelope, PTPO stationery, etc.
Perhaps your inquiry may "fire up" some other postal stationery collectors to have a bit of a discussion about the cards in this forum.