Arthur Penhaligan is just a mortal boy, but a Trustee's plot backfired and he was declared the Rightful Heir to the Architect's Will. Although he himself has no magical power or physical abilities (particularly compared to the angelic Denizens), Arthur has managed to secure three out of the seven parts of the Will through good sense, hard work and a little help from the Will. But the remaining Trustees (also known as Morrow Days, each particularly powerful on their day of the week) are each stronger than the last, and begin to look on Arthur as a serious threat.
Mere minutes after he takes over the Border Seas from Drowned Wednesday, Arthur is drafted to be part of Sir Thursday's army. He is forced to leave behind the magical Keys he's collected, as well as all his friends and allies. Dame Primus, who is the anthropomorophic manifestation of the first three parts of the Will, is left in charge of Arthur's territories--but Arthur isn't sure he can trust her. And back home, in the mortal realm, a Nithling has taken Arthur's form and holds his family hostage. Arthur asks his mortal friend Leaf and not-quite-mortal friend Suzy Turquoise Blue (possibly my favorite character) to stop the Nithling. And then Arthur must march off to training and to war.
The worldbuilding and plot have been getting progressively richer and deeper with every book. This is the best yet. The magic is both interesting (an army that communicates using little army men, and fights on a land made of ever shifting-tiles that each stretch a day's ride and are nearly indistinguishable from mortal realms) and scary ("washing between the ears" sounds like a mispronounced threat of cleanliness--until one realizes that the smiling creatures with rubber gloves on are going to wash one's *mind* clean of all knowledge, rather than one's body). Arthur, Leaf and Suzy are in the most dangerous predicaments yet, not least because the infrastructure of the magical House is breaking down and their own allies are stretched too thin to be much use.
Even if Arthur does manage to get Sir Thursday's Key and portion of the Will, he might not be able to use it--because every time he does, he becomes a little less mortal and a little more like a Denizen. I've seen other reviews that don't understand his reluctance to become a Denizen, as they're stronger, taller, more beautiful, and can live forever (unless attacked by Nothing). But Denizens, for all their power, lack free will and imagination, and can never venture into the mortal realm. If Arthur uses too much power, not only will he lose the empathy and freedom of thought, but he'll also lose touch with his adopted family--forever. But if he doesn't use his power, he might very well die, and doom all the realms to a tyranical clockwork existence. It's a hard balance to walk, particularly in the midst of a warzone.
I tore through this book, and started reading the next immediately!