I've held a few prayer books in my time, but none like this one.
This unusual Italian ‘prayer book pistol’ was custom-made for Francesco Morosini, the Duke of Venice (1619-94). The book, which also contains a few prayers, was made sometime in the 17th century.
It was designed in a way that it fired only when the book was closed, by pulling the silk 'bookmark'. When the book was closed, it was impossible to make out the outline of the gun. This makes historians believe that it was most likely used for personal protection.
Clancy's comment: I wonder under what circumstances it would be used? A poor sermon?