The Light of Day by Eric Ambler

Small-time hustler Arthur Abdel Simpson ekes out a living in Athens by robbing gullible tourists. But when an attempted theft backfires, he finds himself out-smarted and blackmailed into driving a highly suspicious car across the border to Istanbul. Then the Turkish secret police get involved, and Simpson becomes embroiled in something far deeper, and more dangerous, than he could imagine. Featuring a heart-stopping jewel heist, this compulsive, morally complex thriller became the basis for the classic film Topkapi.

'He took the spy thriller out of the gentility of the drawing room and into the back streets of Istanbul, where it all really happened' -- Frederick Forsyth