The Paletti Notebook - Darren Priest Mysteries, #3

Florence, 1553. An assassin sent by Arma Dei disappears into the night, with him a collection of heretical art and writings. For five centuries the satchel, containing sketches and writings by Leonardo da Vinci, Michelangelo and many others, is pursued by kings, popes, princes, and scavengers. The Paletti Notebook lives on through rumor and legend, which also suggest that it contains the infamous Gospel of Matthias, thought to be lost forever.
In modern day Vienna, a bank manager discovers World War II-era photographs that hint to the collection's existence, sparking a new campaign by opposing forces to find the Paletti Notebook and take possession of its crucially important contents - by any means necessary.