Book 16 will be titled PEACE TALKS, and will probably be one of the more supernaturally violent books to date. :)
The news was confirmed on 23 May 2014 on Jim's website:
The sixteenth book in the Dresden Files series will be called Peace Talks.
It’s still way too early for blurbs, preorders, release dates, and the like, but we’ll announce further details as we have them!