Every so often a book starts with a question I can't shake. This one began with a simple accounting problem. When Alexander VI died in August 1503—Cesare bedridden with the same fever that killed his father, the cardinals already circling—the Borgia treasury was among the most coveted in Europe. Within weeks, much of it was gone. Some of that dispersal is documented: seized by rivals, spent on Cesare's dwindling army, quietly reclaimed by families the Borgias had bled. But there are gaps in the record that historians have argued over for five centuries. Money and relics don't simply evaporate. Someone moved them.

That "someone" became the Sisters of the Sacred Vigil—a sodality founded in the chaos of 1503, still keeping its promise more than five hundred years later, and led today by a woman who learned her tradecraft in Italian military intelligence rather than a convent. I wanted an antagonist who wasn't a villain in the cartoon sense, but a keeper of an oath so old that its original purpose had long since curdled into something else.

And of course this was the book where Michael and Hana finally marry—at her villa in Èze, on the Côte d'Azur, with Karl standing as best man, Sister Teri as bridesmaid, and Cardinal Severino coming out of his monastery to officiate. I've had that scene in my head for years. Writing it while a conspiracy closed in around the wedding party was the most fun I've had at the keyboard in a long while.

So, a few things I'm curious about:

1. The Sisters. Mother Superior Agnese Visconti believes she is protecting something sacred. Did you find her sympathetic at any point—or did her military past make her impossible to trust from the start? Would you have preferred a more conventionally sinister order?

2. The wedding. Twelve books is a long courtship. Did the marriage feel earned? And for readers who followed the papal decree allowing Michael to remain a priest and Prefect—does that arrangement work for you as the series moves forward, or does it raise questions you'd like to see the next books confront?

3. The ledger and the reliquary. The investigation turns on a hidden account book and a relic that shouldn't exist. I try to keep these plausible—every archive in Rome has objects whose provenance is a shrug. Which discovery in the book made you stop and think "that could actually be real"? Which strained your belief?

4. The clock. Longtime readers may have noticed this is the first book I've written using 24-hour time throughout. Did you notice? Did it change the feel of the pacing, or was it invisible?

And one open question, because I ask it of myself after every book: what do you think the Sisters were really guarding? Not the object—the idea behind it.

I read every reply here. Pull up a chair.

—Gary