Spells and Bones by Mac Flynn

Millie and her wealthy protector Count Benjamin Castle have their first adventure behind them, but troubled seas lay ahead in the land of magic and mayhem.
Millie finds herself wondering about the unique staff in her possession, and how she could master its supposedly powerful magic. Castle suggests she find herself a teacher for its flute form, and an opportunity presents itself when a well-known band returns from a long tour. They venture out to make inquiries, but their journey becomes a winding path of relatives and revenants as a strange horror rises from the ghoulish depths of the old city.
Their troubles are complicated by their budding romance, and Millie’s inexperience in a world filled with fantastical creatures and some rather shady characters. One of those characters is the seller of a popular card game among the youth. The parents consider the game a less frightful diversion than the odd music band, but something seems amiss to Millie. She can’t put her finger on it until her curiosity gives her more than she bargained for.
Together Millie and Castle must wade through ghouls, graveyards, and groupies to discover the truth behind the sinister magics cropping up around the city, before they’re both buried by their latest, and maybe last, adventure.