Christmas Holiday by Somerset Maugham.
One of the lesser known works by the author of Of Human Bondage, this novel tells the story of a young English man's visit to a ne'er do well friend in Paris.
On his brief holiday he manages to meet and fall in love with the prostitute wife of a convicted, jailed murderer.
A grim take on the "innocents abroad" theme, and while the writing isn't up to the level of Of Human Bondage or The Moon and Sixpence, it's an excellent nove, later made into a movie starring, of all people, Gene Kelly and Deanna Durbin. The plot was drastically changed, so the movie doesn't resemble the book too much, but that's Hollywood.