I read the first three of Daniel Silva’s thrillers starring art restorer/spy Gabriel Allon, loved them, and dipped regularly into the ongoing books over the years. Number 21, ”The Cellist” illustrates why long series should be killed off before stagnation sets in. This time, when a Russian oligarch is murdered in London, Allon digs for the truth when no one else wishes to, and soon comes upon a female cellist and banker who might hold the key to justice., The premise, though by now hackneyed, holds promise, but the author seems to have misplaced the skills that deservedly earned him bestsellers. So many ancillary characters have entered Allon’s orbit over twenty books that the author is now forced to spend pages setting the scene. Wads of exposition unfurl the plot. And the once-nimble style is now facile (though sharp dialogue remains a strength). Read the first three, not this one.