Merb is definitely the next big thing in the Ruby world. I don't think that it will ever displace Rails as king of modern web frameworks, but Merb's popularity for smaller web projects and simple web services is hard to ignore. By signing Foy Savas to write The Merb Way, I am hoping to continue the Addison-Wesley Professional Ruby Series tradition of giving readers the best possible information from experienced practitioners.
Foy has been actively involved in the Merb open-source ecosystem for over a year, particularly with DataMapper. He uses Merb in his day-to-day activities as a full-time application developer and is an enthusiastic evangelist for the framework. He will be working on The Merb Way book full-time, and we hope to have it published early next year.
Incidentally, you can meet Foy in person in Boston on November 17-20 at the Addison-Wesley Voices that Matter: Professional Ruby Conference where he will be introducing the audience to the wonders of Merb development.