Blue noon - Scott Westerfeld

The Midnighters have emerged victorious from their greatest challenge. They now know much more about the secret history of Bixby and, with the halfling dead, the Grayfoots' link to the darklings has been severed. But the cost is high. Rex's horrific experience in the desert has left him damaged, painfully suspended between light and dark. Melissa's violation of Dess's mind and the shameful revelations of her past deeds have shattered the uneasy bond among the five teenagers. What they need now is some time to heal, but what they get is the surprise of their lives when the blue time arrives in the middle of the day.

It seems the walls between the secret hour and real time are crumbling, and soon the dark creatures will break through to hunt after centuries of waiting. And as if that wasn't enough for Jessica to deal with, her little sister, Beth, is becoming more and more determined to crack the secret of midnight - a goal that could have consequences more dire than she can ever have imagined.