Despite being published in the nascent days of the internet, this book would still be of great use to anyone writing detective fiction. I can even imagine such a person keeping this book by her bedside, dipping into it or reading from cover to cover, gleaning, along the way, basic information on the history of the police in the U.S. and England, mystery tropes such as the gentleman thief and the locked room mystery, and the origins of terms like red herring or whodunit. The entries are signed and contain references for further reading.
I spot-checked Wikipedia and found the information in the Companion generally superior. The book scoops Wikipedia by four years on the etymology of whodunit, with a reference to a usage in 1930. It's got carefully researched entries on topics like the spinster sleuth and the slicks, which would be difficult or impossible to find online. Most important, the Companion is slated to writers. The exact kind of information a writer might need on smuggling, sex crimes, or bribery are in the Companion; I found no online source with succinct, writerly overviews on these topics.
Winner: the print book.