I'd forgotten just how good these books are. Mister Monday is a strong start to the series. It introduces the House and gives an idea for it's workings in a way that makes you feel like there's more to learn, but you're not lost.
Arthur is a great hero. For starters, he has asthma which is pretty unique. It certainly causes problems throughout the story, but it doesn't make it seem as if asthma is Arthur's Achilles heel or something. I also like the fact that he wants to help these people and this world he's only just learned about, but he doesn't want to be a hero. He doesn't want to be the heir or fight the Morrow Days or be turned into a denizen. He wants to stay human and live his normal human life.
The Nutshell: If you enjoy middle grade adventure stories, I highly recommend the Keys to Kingdom series.