What's the Name of That Book??? discussion

I read this in the last 5 years. I think it was set in Canada, and the nebbishy protagonist was caring for a young girl (his niece, maybe), and they had a cat who kept escaping to other apartments in the funky old house that they lived in. A hired thief or killer had come up from the U.S. to get something from (or kill?) the old neighbor upstairs, and the young girl spots the man at a diner. I think she loses something that he picks up. It was light and fun, and I think some romance develops with another tenant.

SOLVED: Windfall, by Byron TD Smith.