Lawrence Block once said there were only two kinds of mystery novel. One with cats, and one without cats.
The cat is sort of the standard crittur for mysteries; look at the success of the Cat Crimes series of anthologies. A lot of mystery booksellers keep a cat in the shop. I asked one shop owner why, and she said, "It's expected."
Carolyn Hart has been around for a long time; you must have picked up one of her "Henrie O" books, a series with a lady detective "of a certain age". (She had an earlier series about a married couple that ran a mystery bookstore.) One book of the "Henrie O" series was made into a TV movie three or four years ago, doubling as a pilot. It wasn't very good; in fact, it sank like a stone. Barbara Eden was completely wrong in the lead; but she's the one who took out the option on the book (looking for a new TV series for herself). William Shatner was in it, wearing a very strange bathing suit.