Arthur Penhaligan was only chosen as Heir to the Architect's Will because he was about to die of a brutal asthma attack. But to the surprise of the magical beings known as the Morrow Days, he not only survived, he also managed to wrest control from Mister Monday and Grim Tuesday. Each of his triumphs only serve to whip his foes into a greater froth of rage, and they are drowning him in paperwork, pursuing legal action--and threatening his mortal family. Arthur has barely returned from his adventure against Grim Tuesday when he's magically whisked back to deal with the next of the Morrow Days: Lady Wednesday. Unlike her fellows, Wednesday would love to relinquish her power to Arthur, but a fiendish pirate named Feverfew has control over it. To save the Border Sea from being eaten by the voracious Wednesday (who no longer has control of her appetite), Arthur must defeat Feverfew--all while he has no magical power, a broken leg, and asthma.
This might actually be my favorite of Arthur's adventures yet. Nix is gifted at creating fantastical magic and creatures without ever seeming derivative or cutesy-clever. There's danger in the magic, particularly to the mortal main character, who is surrounded by magical creatures who don't need to eat, drink, or breathe, and can even survive beheading. Even Arthur's allies are a bit casual about his bodily needs (like breathing). But there's also wonder and imagination in Nix's world: a counting house is turned into a sailing ship, but remnants of old magic remain and so every day at teatime cups of tea and biscuits appear out of the sea in the exact location of the former tea-room; every inch of a sorcerer's skin is tattooed, and the tattoos reflect his current state (broken masted hulks sinking when he's depressed that abruptly gain masts and bouyancy when Arthur asks him if he's really recovered enough to help); the rats the Pied Piper piped away from the mortal realm have become Risen rats who will help anyone in exchange for new information...it's all wonderful and memorable, and viewed through the eyes of a very nice, relateable boy.
It's all a bit mad and topsy-turvey and out of Arthur's control, but he's smart, good-hearted, and hard-working, so it might turn out all right after all. It's a bit like a mash-up of Treasure Island with Alice in Wonderland--very fun!