(Solved) Looking for an older book with magic

Reading "Angels' Blood" reminded me of a series of books I read quite a few years ago.  I think the author's name was "Swenson" or something like that.  It is a fantasy about a h who goes to work for a large company that deals in magic in, I think, New York.  They hire her because she has the ability to see what people, like fairies and other characters, really look like, as they have the ability to change their looks, and evil characters can mask their true identity.  And there is an upstanding H, and another character who is, I think, suppose to be Merlin come back from the past.  I'm very shaky about these facts, but it was a cute series of stories.  I should take better notes of my reading.  Any ideas?