Serafina and the Black Cloak by Robert Beatty
My rating: 4 of 5 stars
Serafina lives in the basement of the Biltmore Estate, hidden from the rest of the inhabitants by her father; one day she ventures upstairs and into the life of Braeden Vanderbilt, "son" of the house (nephew of the owners, who act as his guardians) and seemingly unflappable in the face of disappearances and mysteries. The two strike up a friendship and that partnership leads to Serafina's eventually finding who has been causing the disappearances of several of the Biltmore's visitors, in addition to learning more about her mother.
The folklore part elevates this a little beyond the normal "servant girl/rich boy" relationship without overwhelming the entire story.
ARC provided by publisher.