The determined snoop would not be deterred by this little lock on his sister’s diary. We are all aware of the impulse to keep this stuff private, and the little lock may at least send a clear signal to those not utterly lost to conscience’s twinges. Apart from this application, locks on books tend to be a security measure rather than a deterrent to reading.
Medieval books written (or printed) on parchment usually had a clasp to keep them closed. This clasp isn’t there to deter reading. Parchment will swell in damp air, and being held tight at the spine by the binding will cockle and fan out unless restrained. Erik Kwakkel explains this, and more in his post “Hugging a medieval book”, on body analogies in book making.
Early libraries often chained the books so that readers wouldn’t walk off with them. All those chains in the Hereford Cathedral library must have led to a lot of “Sh, Sh”s from the librarian.
A post from Medieval Fragments shows other pictures.
This video (sent by Ink, Bits, & Pixels) has a clasp with a more serious intent. I’m not sure why you shouldn’t be allowed to read this book if you look too enthusiastic though.
Later: Erik Kwakkel now has an extensive account of locked books on his blog, Medieval Books.