Difficult — and delightful — as it is to believe, P.D. James keeps getting better. Her new book, “The Murder Room,” might be the best mystery novel of 2003.

There have been instances in James’ books when the last murder victim seemed an unnecessary excess or a corpse was made to look grotesque. “The Murder Room” has no such jarring elements, nor is the mood as dark as it is in some of her books.

The prose is smooth and unhurried, but never uninteresting — it might remind some readers of Jane Austen’s. Sights and sounds are detailed so that the reader can place himself in the scene.James knows each person in the book, through and through. All, even the simplest among them, are revealed as fairly complex.

The place is a fictional London museum, the Dupayne, dedicated to the years between the two world wars. It’s small and obscure. The room dedicated to the era’s high-profile murder cases is a favorite with visitors.

James begins with a chapter devoted to each person connected with the museum — the founder’s three children, a book researcher who uses it, and the staff. For the museum to stay open, all three Dupayne siblings must sign their approval. Everybody concerned wants it to continue except the youngest brother, a psychiatrist.

In her previous book, “Death in Holy Orders,” James’ detective, Adam Dalgliesh, fell in love with Emma. James neither neglects nor dwells on the possibilities of where this will go. But the question is never forgotten.

This is a book to savor. Instead of being a page-turner that makes the reader eagerly race through the story, this is a page-holder, with writing so felicitous that the reader doesn’t want it to end.