Check the wiki, there are links there to places like the LOC, NYPL, etc that have good online rare book sections...
Also, for something hidden, this isn't exactly what you were looking for but, you might want to take a look at
fore-edge paintings. His are obvious new work, but if you google you can find originals... The images only appear when the pages are fanned...
I guess another 'hidden message' old text would be the Voynich manuscript... If you can decipher that one I think the CIA would have a job waiting for you...
-MJ