Publishers like to use French flaps to give a paperback a de-luxe look, and justify a higher price. They are quite fashionable these days as are all sorts of embellishments to our books. (Once upon a time there was a more general consensus that the content was what would sell the book, though I guess even back then we did try to make our jackets pretty.)

French flaps are just like the front and back flaps on a book jacket transferred onto a paperback cover. They do increase your costs, partly because they use more cover board, but mainly because they split the trimming process into two steps. The fore-edge of the book block has to be trimmed before the cover goes on — if it wasn’t you’d end up with handy bookmarks in the front and back of the book when the guillotine chopped off the edges. Then after the cover has been applied to the spine, the book gets trimmed again to chop off the top and bottom margins. Not every book manufacturer is set up to provide this option, so price and scheduling can be issues. There’s always a risk of over-trimming the fore-edge so that the cover overlaps a bit too much. The opposite effect, leaving the white pages sticking out beyond the cover is so awful that the safety margin is moved inwards. Be it said, the manufacturer of the Very Short Introductions series does a pretty good job in this regard.