I picked up Seven for a Secret at a conference not realizing that it was the second in a series. I probably would have understood more about the characters had I read The Gods of Gotham first, but I don't think that not having read it affected my enjoyment of this book.
Timothy Wilde, copper star of the newly formed NYPD, is approached by a woman named Lucy Adams whose son and sister have been abducted by slave catchers in spite of being free natives of Albany. Rescuing him with the help of his friend Julius Carpenter and the New York Committee of Vigilance is only the beginning for all of them. Timothy is pulled into a murky world of slave traders and politics, with his brother Valentine assisting him only so far as his interests don't disturb the Democratic party machine.
This is a dark, twisty tale full of deceit and vividly drawn characters, set against New York City at a time when the city was filling with refugees from the Irish Potato Famine and tensions over slavery are simmering.