The Columbus Letter by Susan Flowers

Iris Dittmeyer’s life is losing its luster. The retired widow needs a distraction after her granddaughter’s daycare enrollment means she's no longer needed for babysitting. The discovery of an acclaimed Columbus letter in a battered filing cabinet provides just the mystery she’s been craving. But when her new diversion turns deadly, it’s up to Iris to find the killer—before her family becomes his next target.

Giovanni di Caro claims knowledge about historic documents and, and with a little coaxing, he agrees to assist Iris in the investigation. The charming Italian considers himself a craftsman, but the international authorities have another name for Giovanni. He’s the notorious forger of Galileo’s Sidereus Nuncius.

They’re a mismatched duo from opposite sides of the world. She’s an early riser, a tea drinker, and a busybody. He’s a night owl, coffee devotee, and country music fan. Their relationship?

It depends on who you ask.