Neatorama finds this item which
sold at auction.
The name of the book is
Historia Universalis, which makes me wonder if it was chosen for its irony. The book is a fake, obviously, which folds out to construct the device seen here. You'd put a chamber pot underneath, and then you'd sit and then, I suppose, you'd have to aim. Really well.
Another historical skill lost to invention.
The auction site says "An unusual example of the use of the book form to disguise travelling personal furniture, probably for use on the military field." Ah, marksmanship.