The Shadow Thieves by Anne Ursu.

In which Charlotte Mielswetzski lives her life (being followed by pale-faced yellow-eyed bald men in tuxedos). She finds a kitten (or the kitten finds her..?) and names the cat Mew. Then her cousin Zee comes over from London and tells her about how all the kids back in London are getting a mysterious illness and Zee thinks he's the cause! When the kids in Charlotte's school start getting sick and start losing their shadows, they (Charlotte and Zee) go to find out why. In the middle of the night a raven comes and takes them to the mall (of all places) that takes them though a door to that leads to the Underworld. There they meet nursery-rhyming Harpies, a fruit-roll-up -loving Charon, and an evil guy named Philonecron. Can they stop this hot-shot who thinks he can take over the Underworld and save the dead?

Here's something funny: Philonecron means 'lover of death.' That makes me laugh. On the first page the author says: "Pay attention. Watch carefully, now. Look at the sidewalk, there. See that girl - the one with the bright red hair, overstuffed backpack, and aura of grumpiness? That's Charlotte Mielswetzski. (Say it with me: Meals-wet-ski. Got it? If not, say it again: Meals-wet-ski. There. You thought your name was bad?)" I liked the Harpies who sang (my favorite) "Twinkle twinkle, little man, I wonder how you'd taste with jam, Chained above the world so high, Like a lamb chop in the sky. Twinkle twinkle little man, wonder how you'd taste with jam." Ha ha ha ha!

This is book 1 in the Cronus Chronicles series.

Recommended for people who like: shadows, brave heroes and heroines, and a great story!