The Detective's Assitant by Kate Hannigan

Recently orphaned Cornelia (Nell) gets sent to her maiden aunt who is her only remaining relative.  The year is 1859 and Aunt Kitty is the first and only female detective in the Pinkerton Detective Agency.  At first Aunt Kitty wants to find a respectable boarding school for Nell, to get her out of her hair, but gradually she comes to realize that Nell is not such a bad detective operative herself.  Together they solve some pretty tough cases, but the most difficult mystery of all--what really happened when both Nell's father and Kitty's husband were killed--is the one Aunt Kitty is not quite ready to tackle.

This was a cute historical mystery.  The characters of both Nell and her aunt are endearing and well rounded and their relationship develops in a charming (if predictable) way. It turns out that much of the information about Aunt Kitty is based on a real historical person, Kate Warne.  She was, indeed, the first female detective, and the cases they solve in the book are ones that Pinkerton wrote about in his memoirs.  (359 p.)

So here is the question.  What kind of genre sticker do I put on this book, historical fiction or mystery? (I have just started a project to put genre stickers on the J Fiction section in my library and am finding that designating genre is trickier than I thought.)