There's no new Dark Tower novel coming out in 2015. Steve has mentioned wanting to write about the battle at Jericho in recent interviews but he hasn't written that yet.
You know, as much as that is a watershed moment in the DT story, I almost don't want him to write it. Right now, it's a mythic story: How Mid-World Fell. I'd bet that every reader has a version in his or her own head of want happened there, necessarily misty and vague, but there. It would be a terrible task to live up to those mind movies. I liken it to the picture Mr. King drew in Danse Macabre (though there he was talking about monsters): If you can sustain terror--we never see the shape behind the door--it's the finest way, because once you open the door the reader can relax and say something like: "Oh, it's a 50 foot monster. That's bad, but what if it was a 500 foot monster!" Same thing here. "Oh, so that's how Cuthbert fell! That's pretty awful, but I was imagining______. That would be much worse." At best, people shrug; at worst they are disappointed.
I know a lot of people would disagree with me, but I calls 'em how I sees 'em.