This one seemed promising at the start - the scenes in Florence were entertaining and the descriptions pleasing. By the time the action reached Naples the whole thing had become ludicrous. After surviving a shipwreck by floating up to a grille, the two main characters have a few adventures and then go into a church looking for 'a lady'. They see a carving and realise that the numbers above it are significant (I won't spoil the story but it was a stroke of genius which caused one of the characters' eyes to glow blue). They go to a different carving, tug away at a piece of the stonework then press the numbers above it. Abracadabra, a secret passage opens. I was baffled as to how they'd done it and had to re-read it several times. I still don't know.
There seems to be a glut of these historical-mystery thriller novels at 99p or less on Kindle. In my opinion too many of them have ludicrous plotlines and shall we say experimental grammar - they seem to have been written by non-scholars who are trying to jump on the Dan Brown bandwagon. This one was a little bit more interesting and quite a lot better written than some of the others I've read and it may well appeal to many readers. But I gave up when the two main characters, a prostitute and a monk, ran down the aforesaid secret passage leaving a murderous leper with silver eyes wondering where they'd gone. I no longer cared.