The death of former KGB agent Alexander Litvinenko -- poisoned by polonium in a cup of a tea in an English hotel six years after he had sought political asylum there -- has been covered in at least four books of which I'm aware. This strikes me as the best of the bunch so far, although the definitive history of Putin's Russia has yet to be written. (Perhaps the reason for that can be found in some of the contents of the book itself, including the events leading up to Litvinenko's death.