The 40-year-old actor discussed the sweet gesture and other details about his romance with the 44-year-old actress in the April issue of Cosmopolitan U.K.
"I wrote her a book as a gift for our first anniversary," he said when asked about the most romantic thing he's ever done for Vergara. "It was about how we met -- as well as our courting -- and was about 40 pages long."
Manganiello and Vergara married in Palm Beach, Fla., in November 2015 after a whirlwind courtship. The couple started dating in mid-2014, which the actor recalled in the interview, and got engaged the same Christmas.
"I was on a press tour when I found out Sofia was single [after her split from Nick Loeb]," he said. "My friend is an editor, and told me she was about to make the announcement. I got her number from [her Modern Family co-star] Jesse Tyler Ferguson, then flew to New Orleans to take her on a date."
"Sofia and I have always put each other's well-being first, and that's how I knew she was the one," said Manganiello.
The Magic Mike XXL actor had shared similar sentiments about Vergara in the March issue of Hola! USA. He acknowledged he and the Colombian-born actress "got married very fast," but said they're "completely compatible."
"We realized very early on that we had to put the person's happiness and well-being ahead of our own," the star said. "Once you realize that kind of trust has manifested, you hang on for dear life. Or in my case, you go and buy a ring."