Book 19 - Mirror Mirror, an alternate timeline involving Harry Prime getting swept into another universe by his evil alternate self from that timeline. Evil Harry's been killing other Dresdens and ends up biting off more than he can chew with HP. There's a hint that alternate Mab might rope HP into working for her while he's there. Jim said things went to hell faster in that universe. May be foreshadowing the BAT. Timeline diverges in Grave Peril.