Listened: Death's Door by James R. Benn, 2012 (I think), downloaded form Overdrive.
The problem with writing historical novels, especially something from World War two which still has tons of amateur historians in tune with the events, is that so much of write you write can be total bullshit. Eisenhower sending an agent into the Vatican during the Nazi occupation of Rome during the battle of Anzio just to investigate the murder of of Catholic Bishop who was a classmate of the OSS's "Wild Bill" Donavan? Bullshit to the fifth.
I liked it anyway and enjoyed the whole damn book. Even Billy Boyle's whiny introspective parts.
Anyway. Billy Boyle is in Allied occupied Italy mourning the absence, and likely death, of Diana who was in Rome, undercover as a nun, and captured by the Krauts. Boyle has been AWOL and sad-eyed when he and his Polish Army pal Kaz are picked up by MPs and delivered to a Limey installation. Boyle has been tasked with going to the Vatican and investigating a Bishop's death. The Vatican is under a lot of pressure from all sides and their treaty with Italy, and therefore the Germans, is constantly under threat. There is a good chance the Germans will kidnap the Pope and move him North into "protective custody".
Boyle is flown a little north himself and put onto a boat on the East coast of Italy. The boat captain is Sterling Hayden. Hayden delivers Boyle and Kaz to the shore. Boyle and Kaz are disguised as priests, make their way to a trainyard, kill a Kraut, are secreted into a train car, smuggled into the Vatican.
Boyle starts asking questions. A fugitive Jew was arrested for the Bishop's murder. Many Vatican authorities are pro-fascist. The Vatican is filled with refugees, escaped POWs, and Jews. Boyle wants to try and rescue Diana from a Nazi prison.
Things happen. Vatican scenery. Vatican reality in WWII. Dirty, rotten, stinking, filthy Nazis. Doyle tries to rescue Diana and is extorted by German intelligence officer from previous books. German wants to kill Hitler and needs peace agreement with the western Allies.
Everyone lives happily ever after except for some dead priests.
1. This could read as an apologist reaction to criticisms of the Pope and Vatican during WWII. I still read occasional articles about how the church did not do enough during the war. But, the Vatican did quite a bit. What's more, the Vatican is pretty dang tiny and without much pull with the dirty, rotten, filthy, stinking Nazis.
2. To betray their neutrality was their destruction - and then they could do nothing. The Pope and Vatican was between a rock and a hard place. To do Christian work the priests would find the cracks in the cracks in the hard place and slip around the rock.
3. That was a poor analogy.