The Average of Deviance by K. Simpson

After 'Several Devils,' Devlin Kerry and Cassandra Wolfe aren't just friends anymore, but they'd rather not advertise. Trouble is, they're already in advertising, where secrets are practically illegal. Word is out... so to speak.

Worse, Dev has a demon. Cassie says it's her own fault for being celibate for years ('You can't go around not sleeping with people and not expect to get into trouble'). They thought they'd exorcised the demon, but Monica is back. And this time, she plans to stick around.

This sequel picks up where 'Several Devils' left off, as the staff of J/J/G Advertising tries to get back to normal the morning after a wild Halloween night. But every day is Halloween at J/J/G, the way these people behave, and everyone there is a devil of some kind. The last thing Dev and Cassie need is an actual demon -- a chance for mischief that Monica can't resist. Thanks to her, they have trouble coming from all directions: co-workers, clients, TV reporters, the family-values crowd and even a possum.

Worst of all, a new co-worker may not be what she seems. Dev and Cassie are about to learn the true meaning of 'hell to pay.'