The Rook by Daniel O’Malley
Loved this book! Granted it's not perfect - there's a fair amount of info-dumping made palatable by the whole amnesia bit. It's kind of a paranormal, sci-fi-ish spy thriller, with a dollop or two of humor.
As the blurb says, Myfanwy wakes up with no memory surrounded by dead bodies. She is guided back into her life as one of the heads of a secret paranormal agency by letters she wrote to herself, having known she would lose her memory thanks to the warnings of a variety of psychics, including a duck. So she fakes her way, but also discovers she has an AWESOME power that the old her barely made use of. She's a character to root for, the underdog due to her amnesia and that people underestimate her and never truly respected the old her. Oh and she's facing an enemy who has been waiting for revenge on England for centuries and has all kinds of yucky, nasty and dangerous things/people/fungi...