The Shadow Thieves (Cronus Chronicles)

It's not the very cute kitten that appears out of nowhere. It's not the arrival of her cousin Zee, who believes he's the cause of a mysterious sickness that has struck his friends back in England. And it's not the white-faced, yellow-eyed men in tuxedos who follow Charlotte everywhere. What's so extraordinary is not any one of these things. It's all of them.

When Charlotte's friends start to get sick, Charlotte and Zee set out to find a cure. Their quest leads them to a not-so-mythical Underworld, where they face Harpies that love to rhyme, gods with personnel problems, and ghosts with a thirst for blood.

Charlotte and Zee learn that in a world overrun by Nightmares, Pain, and Death, the really dangerous character is a guy named Phil. And then they discover that the fate of every person -- living and dead -- is in their hands.