The Poe Estate by Polly Shulman

Because of financial troubles, Sukie's family moves in with an elderly cousin who lives in a creepy old house. Sukie soon discovers that the house is haunted, but that is nothing new to Sukie. She has been haunted by her deceased sister since her death years earlier. The house ghosts tell of a lost treasure and soon Sukie and a new friend, Cole, are on a quest to discover the treasure and save her family from financial ruin.  But they can't do it alone.  They need the help of the New-York Circulating Material Repository and its mysterious archivist, Elizabeth Rew.

This is a companion book to The Grimm Legacy, and The Wells Bequest, but I didn't realize that when I read it.  I thought it was the first in the series, and it works just fine that way.  If I had read the first two books, I would have recognized Elizabeth Rew, who is the main character of Grimm Legacy as soon as she appeared in the story, but Shulman gives enough background that it was just fine to read this one first. The premise of all three books is interesting and original, but the book's real appeal comes from the strong and likeable characters. Sukie has a kind of "Harry Potter" feel as she gradually discovers her own powers and her place in her family history.(259 p.)