Butcher said on
his recent Reddit AMA that it would be the title, but I haven't seen an "official" announcement yet.
It's about as "Official" as these things go. Jim's been using the "PEACE TALKS" title for Book #16 during the current Book Tour.
FYI the current title for Book #17 is MIRROR MIRROR (Which should hold firm as long as the story doesn't swap position in the current order which has happened a few times so far.)
Incidentally, we know more about Mirror Mirror than we do about Peace Talks. All we know about Peace Talks is that it's going to have the White Council, and Ivy, and more supernatural violence than any previous book. And presumably some sort of talks.
Mirror Mirror, on the other hand, we know its caused by Mirrorverse Harry summoning Prime Harry in order to kill him to as part of a faking his own death scheme. And he bit off more than he could chew by grabbing the freaking Winter Knight instead of some other more vulnerable alt.
We also know that the two Harrys diverged at the end of Grave Peril. Presumably, Mirror Harry let Bianca take Susan to prevent the war. And that led to a whole lot of bad choices.