Cabal by Michael Dibdin (Inspector Zen Mystery Series #3)

'As you may have gathered, there was a suicide in St Peter's this afternoon. Someone threw himself off the gallery inside the dome. Such incidents are quite common, and do not normally require the attention of this department. In the present instance, however, the victim was not some jilted maidservant or ruined shopkeeper, but Prince Ludovico Ruspanti.'

When, one dark night in November, Prince Ludovico Ruspanti fell a hundred and fifty feet to his death in the chapel at St Peter's, Rome, there were a number of questions to be answered. Did he fall or was he pushed? Inspector Aurelio Zen finds that getting the answers isn't easy, as witness after witness is mysteriously silenced - by violent death. To crack the secrets of the Vatican, Zen must penetrate the most secret place of all: the Cabal.