Kindle Teens Daily Deal for December 17, 2012: Sanctum (Guards of the Shadowlands) by Sarah Fine for $1.99
From the description:
"A week ago, seventeen-year-old Lela Santos’s best friend, Nadia, killed herself. Today, thanks to a farewell ritual gone awry, Lela is standing in paradise, looking upon a vast gated city in the distance – hell. No one willingly walks through the Suicide Gates, into a place smothered in darkness and infested with depraved creatures. But Lela isn’t just anyone – she’s determined to save her best friend’s soul, even if it means sacrificing her eternal afterlife.
As Lela struggles to find Nadia, she’s captured by the Guards, enormous, not-quite-human creatures that patrol the dark city’s endless streets. Their all-too human leader, Malachi, is unlike them in every way except one: his deadly efficiency. When he meets Lela, Malachi forms his own plan: get her out of the city, even if it means she must leave Nadia behind."
4.7 stars on 59 reviews. I'd imagine this one is good for 12 and up.