a deliberate re-dressing by young browsers, who loved watching the shocked reactions of sober readers finding sex manuals in the jackets of bird-watching guides or cookbooks – Jomama

In conjunction with:

But I cannot see what thrills youngsters would get out of the switch because it might be days or weeks before someone comes in and looks at the switched book – TEd

Years ago, I worked in a bookstore that had a similar problem. We found it wasn’t that the kids wanted to play a prank, but that they wanted to look at books and magazines dealing with sex and would hide them in the covers of other books so they could look at them in the store without drawing too much attention to themselves.