O Jerusalem is a book by Laurie R. King and was published in 1999. It is the fifth in the Mary Russell and Sherlock Holmes mystery series.
At the start of 1919, Russell and Holmes leave England for a break from the events in The Beekeeper's Apprentice, and travel to Palestine (modern Israel) at the request of Mycroft Holmes. While disguised as Bedouins, they must piece together a series of seemingly unconnected events, and stop a plot to disrupt the peace process.