Give you two books:
The Last Cato by Matilde Asensi
http://www.amazon.com/Last-Cato-Nove.../dp/0060828587
and
The Day Time was Hacked by Carel Mackenbach
http://www.amazon.com/Day-Time-was-H.../dp/B00375LIF6
Both give you a different look at the Vatican, while not pivoting around a secret which is too far-fetched to be convincing.