This is book number 3 in the The Familia Arcana series.
Finn Graymare is back in the final installment of Randy Henderson's Familia Arcana series, Smells Like Finn Spirit.
Finn's re-adaptation to the human world is not going so well. He's got a great girlfriend, and is figuring out how things like the internet work, but he is still carrying the disembodied personality of Alynon, Prince of the Silver Demesne, the fae who had occupied his body during his imprisonment. And he's not getting along at all with his older brother. And oh, by the way, his dead grandfather is still trying to possess him in order to bring about Armageddon.