I have read all the Gabriel Allon series, and this one must rank as one of the best. In it, our hero, art restorer, Israeli agent and assassin locks horns once again with Ivan Kharkov, the arms-dealing Russian bad guy, from Allon's previous outing, Moscow Rules. Because the story-line follows on from that one, I'd definitely recommend reading that one first, although enough background is given here, if you do decide to jump straight in.
The disappearance of the defector of the title from a London street, triggers concern that Kharkov may be seeking revenge for his downfall in the previous book, and in particular for the safety of Gabriel Allon and his new wife Chiara who have been enjoying their honeymoon in a villa in Umbria. There is good old-fashioned spy-craft here, intricate plotting, much hopping between venues in the US and Europe, and a high body-count so maybe not for the squeamish. However, what makes this series of books stand out for me is the characters, Allon himself and Chiara, Ari Shamron and Gilah, Sarah Bancroft, Adrian Carter, and the rest of them. This is a thoroughly enjoyable series, and I do hope that Daniel Silva manages at least a few more outings for Gabriel and co, before he allows him to put down his gun for good.