Book Advertisement Posters from the 20's ??

I have recently purchased a work of art and when I took it out of the frame...behind the art was 4 hand painted panels of advertisements for books. They appear to be from the 1920's, from the information I've gathered on the books. They are hand painted and have a paper copy of the front cover of each of the books attached. They are in very very good shape...the colors are unbelieveable. The only damage I can see is the edges on a few were rubbed a bit and they have some tape which is water based, which I am going to remove. I know how. The only other thing I can see is that they cut a little off the bottom to fit them in the frame. This doesn't really detract from the beauty of the panels.
I was wondering if you could possibly help me figure out what they are and what they might be worth? I have looked all over the internet and cannot find anything like them. I'm thinking they must be pretty rare.
Thank you!