Amazon have acquired more licenses for the Kindle Worlds program. These are from Valiant Entertainment and from various authors.
Valiant Entertainment Worlds
Archer & Armstrong
Neal Stephenson's Foreworld (the Mongoliad books)
Hugh Howey's Silo Sage (Wool, Shift, Dust)
Blake Crouch's Wayward Pines (Pines and the forthcoming Wayward)
Barry Eisler's John Rain (A Clean Kill in Tokyo, A Lonely Resurrection, etc.)
Amazon have writing guidelines here