A recent Twitter post by Kazu Kibuishi deliberately states that the next book in the series will be published in Fall of next year.
If you weren't aware, Kazu Kibuishi was treated with bacterial meningitis some time in 2012, in which he spent two weeks in the hospital and three weeks recovering at home. I would definitely like the book to be released much sooner, but this is a serious illness and he's lucky to be alive. His memory was affected; however, he's made a gradual recollection since. It's a good thing he's still got a passion for his career, though.