Stephen King is going to write the first episode of Under the Dome's new season that will air next summer.
But a statement from CBS says:
We're excited to tell more stories about the mystery of the dome and the secrets in Chester's Mill
It sounds like the show is staying in the current town for another season. I haven't read the book (no spoilers, please) so I have no idea where it's going but I just don't see how they can drag out the story of Chester's Mill in an interesting way for another 13 episodes.