On the official Mr. Mercedes promo page is a letter from Stephen King. In the letter he announces Finders Keepers, again, but also mentions a book of new stories in the fall of 2015!
Dear Constant Reader,
Mr. Mercedes is the first volume of a projected trilogy concerning Bill Hodges, his friends…and his enemies. The second volume, Finders Keepers, is finished (in first draft, at least) and will be published in the first half of 2015, perhaps followed by a book of new stories in the fall (or not). Anyway, these three novels seem to revolve around the City Center Massacre that opens Mr. Mercedes. Because of that, I wanted to post a complete list of those killed and injured in Brady Hartsfield’s attack. Some of these unfortunates have other parts to play. As always, I’m glad you’re along for the ride, but be warned: there are some sharp turns ahead, and I’m a really crazy driver.
Ms. Mod is a bit more precise:
Collected from previously published in magazines, etc. but complete list has not yet been decided. Whether there are any completely new ones for the collection–to be determined.