(back-to-back) binding. Literally, there are six books in one. You just unclasp the book you’d like to read and the others stay shut. It is dated from the late 16th century from the Rogge Library in Strängnäs.It’s a little bit like the modern e-reader or even one of those books where you get to the end and then flip it upside down to read another book from the other side.
In any event, it’s kind of awesome.