I think it's fascinating that Grisham's publisher is releasing the exact same book in two formats. The Appeal will be published simultaneously in mass market and trade.
Trade is sometimes considered classier than mass market. I've heard that means it might be more likely to be picked by book clubs. Target sells a tons of trade paperbacks, as does Costco, whereas Walmart is more of a mass market kind of place.
Because trade is priced higher, and the royalty rates are comparable, the author makes more money per book sold, sometimes nearly double. But the units sold tend to be fewer.
Have you seen this done before?