An Old Betrayal by Chas Finch

 This is part of the excellent Charles Lenox series of Victorian mysteries, which I have been reading totally out of order as they are available at the library. In this tome, Charles is a member of Parliament, rubbing shoulders with Disraeli and various other bigwigs. His friend John has taken over his private eye firm and asks for a consult as John is under the weather. John has received a letter asking him to meet someone at the restaurant in the train station and the person will have a striped umbrella. Charles goes in his stead, looking for the mystery man--who, he realizes too late, is actually a mystery woman. This sets off a chain of events involving a man masquerading as another, a couple of murders, and an attempt to break into Buckingham Palace. It's all very exciting.