A rather strange structure. At first blush it looks like a classic lapped-component German paper binding with a sort of integral hollow formed through the thick paper spine bonnet/board attachment. In this case, though, either the hollow has collapsed and adhered itself to the spine of the textblock, or it was intended as a tightback structure. In place of the thick paper bonnet is printed waste which seems to turn in at the head and tail. The paper, too, has a rather erratic distribution of colors which I would tend to associate with later styles, but the indirectly impressed circles feel mid-18th c. to me.