You’ve probably come across this. You turn the page and it resists near the gutter. You peer at it and find a little gobbet of slightly stretchy translucent rubber-cement-like stuff sitting along the top edge of the page. You can see it in this picture which I took after ripping off the bit I came upon first.

This dried glue comes from the bond between the endpaper and the case. If the glue’s a bit thick near the middle of the endpaper it’ll just show as a little bump if it doesn’t end up being squashed into even dispersal. But if it’s near the edge the pressure applied in building in will make it squirt out and dribble down the book’s trimmed edge. Because it starts life pretty transparent it’s unlikely to be noticed in production. When it dries it goes about its life’s work of holding bits of paper together.

The glue, as you can see, is however no longer attached to the case. This is because, before the glue has dried, the jacket is applied, which involves opening the case and folding in the jacket flap. If there was no jacket you might find the last few pages still attached lightly to the case.