Seven for a Secret  - Lyndsay Faye
Set in 1845 New York, when the potato famine in Ireland was flooding the city (and a few others in the US) with poverty stricken Irish and at the same time the US hadn't dealt with slavery, yet. It was perfectly fine to kidnap a person of colour and claim that they were an escaped slave, I have seen mention of it before in Barbara Hambly's Benjamin January Series but didn't realise that this happened in New York. Of course Timothy gets involved and of course things get complicated by politics and enemies.

Entertaining series.