The Devil Inside by Jenna Black

Morgan Kingsley is an exorcist called to exorcise a demon from a little girl in Kansas. She begins her ritual only to find herself the one confined after the child touches her during the exorcism. After her fellow exorcist and friend Valerie is brought in to check, Morgan is declared demon-free and returns to her job and lawyer boyfriend, Brian.

Morgan is troubled by sleepwalking incidents that soon escalate into written notes when she is apparently asleep. She discovers that she has been illegally inhabited by a demon, Lugh. Lugh is just as reluctant to possess Morgan. He has been forced into possession by Morgan's brother Andrew, who is legally possessed by the demon Raphael. Morgan has such a strong will that the only opportunity Lugh has to control her body is when she is asleep.

After more than one attempt to murder Morgan, Lugh confesses that he is the recently crowned king of the demons. His brothers Raphael and Dougal are trying to usurp his position as king. Morgan reluctantly seeks the assistance of Adam, Director of Special Forces and demon-possessed and his lover, Dominic, whose demon she has recently exorcised.

Morgan, Lugh, Adam, and Dominic fight to trust one another, to keep Morgan and Lugh alive, and to avoid God's Wrath, an extremist group looking to wipe out all demons, demon possessed, and exorcists.

This was a book that I found difficult to put down. Black found a way to make demons believable and sympathetic. Morgan is a compelling character who you can't help but feel for through the difficult decisions she has to make.