This is book 18 of the "Stone Barrington" series.

This note contains spoilers for the book.

Trying to juggle doing things for MI6 and a rich (lottery-winning) hapless idiot keeps Stone busy.

MI6 is interested in the disposition of an old missing-persons case, somebody who got away from them. Dead, alive under a different identity, and what are they doing just now anyway?

Also he learns to fly a Cessna Citation, not just his turbo-prop.