The Unexpected Guest is a novelization by Charles Osborne of the 1958 play of the same name by crime fiction writer Agatha Christie and was first published in the UK by HarperCollins on 6 September 1999, and on 1 October 1999 in the US by St. Martin's Press.

The book was written following the successful publication of the novelisation of the 1930 play Black Coffee in the previous year.

Like that book, the novelisation is a straightforward transfer of the stage lines and directions of Christie's script into a written narrative.

Osborne chose not to add characters, lines or scenes which would alter in any substantial way what had been presented on the stage forty-one years earlier although minor amendments were made to produce suitable chapter endings.

In the end, they settle on an enemy from the past, by the name of MacGregor, whose son was run over by Richard Warwick, the dead man, several years ago.