Everything you need to understand or teach Afterlife by Marcus Sakey.
Equal parts romance and thriller, Marcus Sakey's novel Afterlife follows the lives and deaths of two FBI agents, Claire McCoy and Will Brody. The two are deeply in love and also hunting a terrorist sniper named Simon Tucks who succeeds in killing Will. However, death is not the end for Will, who ends up in the Echo, a world that acts as a purgatory for people who died sudden and violent deaths. Soon, Simon succeeds in killing Claire as well, and she joins Will in the Echo where they have to learn to survive in the Echo. However, the Eaters - fellow recently dead people - feast on other people in order to get their life forces; also at play are the Elders who manage to survive in the Echo for centuries by torturing those who are still living.