O Jerusalem : a Mary Russell novel by Laurie R. King. Mary Russell and Sherlock Holmes have fled to Palestine on a mission for Mycroft Holmes. Disguised as itinerant Muslims and paired with two Arab spies, Russell and Holmes travel through the Holy Land trying to figure out exactly why Mycroft has sent them. A pair of seemingly unrelated murders sets them on the track of a brilliant and power-hungry killer. Only Holmes and Russell (along with some unexpected allies) can stop their adversary from destroying Jerusalem--if they can get to him in time.

I listened to this on CD and kept wanting to drive so I would know what happens. This is the 5th in the series but in time it takes place after Beekeeper's Apprentice. I have enjoyed all the Mary Russell novels especially this one as you get a real impression on how Russell and Holme's relationship developed.