Book four of the Keys To The Kingdom series.
Under a cut in case of spoilers for this and the preceding books in the series.
Arthur Penhaligon just wants to go home to his family after retrieving the third part of the Will of the Architect and the the Third Key, but things, as ever aren't that easy - his place has been taken in the world outside the House by a Nithling doppelganger which is trying to take over the minds of his friends and family. As if that wasn't bad enough, he discovers that using magic is gradually turning him into a Denizen of the House and that if the transformation progresses beyond a certain point he'll never be able to go home. And then he gets drafted into the army…
But fortunately for Arthur, he has loyal friends, to help him displace the evil double and to regain the fourth Part of the Will and the matching key from Sir Thursday.
Another fast fun read in this splendid series. I like the way the plot is rarely entirely predictable and the characters aren't perfect, but are still likeable.