Dog on it - Spencer Quinn

In this, their first adventure, Chet and Bernie investigate the disappearance of Madison, a teenage girl who may or may not have been kidnapped, but who has definitely gotten mixed up with some very unsavoury characters. A well-behaved, gifted student, she didn't arrive home after school and her divorced mother is frantic. Bernie is quick to take the case - something about a cash flow problem that Chet's not all that clear about - and he's relieved, if vaguely suspicious, when Madison turns up unharmed with a story that doesn't add up. But when she disappears for a second time in a week, Bernie and Chet aren't taking any chances; they launch a full-blown investigation. Without a ransom demand, they're not convinced it's a kidnapping, but they are sure of one thing: something smells funny.