Are you a fan of Sorcery and Cecelia?
Then I might have just the thing for you: Silver in the Blood, by Jessica Day George.
I've barely started it—I'm only about fifteen pages in—but it's already got the same feel, and it's a similar mix of historical-fantasy-humor-mystery, told partly in letters and notes and society columns, partly in diary entries, and partly by a third person narrator. Like I said, I'm only a few pages in, but I'm already in love.
And, to be quite honest, I fell in love with it even earlier—because George had me with her very first few lines:
26 April 1897
Whoever said that travel was exotic and full of adventure clearly has not sailed on the White Lady. Before you worry yourself sick that I am sitting in some squalid cabin, fear not! Of course it is all that is respectable and luxurious, and I would never do something so horribly undignified as become seasick. Fear, rather, that I, your dearest cousin and bosom companion since infancy, shall die of boredom before the trip is even halfway through!
That, of course, was written before the wolf attack! And before Dacia catches her Aunt Kate kissing a strange man in a cloak! And before Lou is approached by a DIFFERENT strange man in Paris! ALL OF THIS IN THE FIRST FIFTEEN PAGES! I swoon.
Oh. ALSO. It doesn't come out until July. Sorry about that. But! If you HAVEN'T read Sorcery and Cecelia, that will be a little something to tide you over until then.