In 1934, one of America’s most notorious prisoners, gangster Al Capone, was carted off from an Atlanta penitentiary to the United States’ most cutting-edge prison: a maximum-security prison on Alcatraz Island in San Francisco Bay. But what did Al Capone do while in the pen for tax evasion and contempt of court? A letter now on auction sums up his leisure activities in two words: prison band.

A 1938 letter from Capone to his son is currently for sale at RR Auctions, a Boston-based auction house that specializes in rare manuscripts. It’s a rare letter, indeed—the missive, which is expected to bring in more than $50,000, gives an intimate look at the daily life of one of the most hardened gangsters in American history.

You’d think that Capone, who was technically imprisoned for tax evasion but spent years as a brutal Mafia boss, might have spent his days in jail trying to get out. But by the time he arrived at Alcatraz, he was in no condition to flee. In Atlanta, where he had served the initial part of his sentence, he had been given special privileges, entertained near-constant visitors and used piles of cash to pay off prison guards. But things were different in Alcatraz. For one thing, Public Enemy No. 1 was suffering from syphilis (some biographers even think that the disease explains some of his erratic, murderous behavior). He was also watched closely by the warden of Alcatraz, who refused to grant him any of those special privileges he had previously enjoyed.

But that doesn’t mean Capone never got his way. According to Don Babwin of the Associated Press, Capone “begged the warden for permission to form a small band.” Capone racked up time for good behavior and took up music, playing banjo in a band called “The Rock Islanders.” The band had a rotating group of musicians who played as a privilege, and if Capone’s letter to his son is any indication, he relished the chance to play both banjo and mandola (an instrument that resembles a large mandolin).

“First I learned a Tenor Guitar and then a Tenor Banjo, and now the Mandola, but for Solo work only,” he wrote to his son, boasting that he could play over 500 songs. Capone even wrote a love song song, “Madonna Mia,” that was published posthumously in 2009.

So what else did Capone do in Alcatraz when he wasn’t, say, dodging a violent stabbing attempt or doing laundry? “My routine here is Morning Yard, I mean the amusement Yard, Baseball, Horseshoes Courts, and Hand-ball courts, Checkers and Dominoes,” he wrote. And he had a few words of wisdom for his son, who was in college at the time. "Junior keep up the way you are doing, and don’t let nothing get you down. When you get the blues, Sonny, put on one of the records with songs I wrote you about,” he said. There isn’t much I can write, but chin up, always.”

He signed the letter “Love & Kisses, Your Dear Dad Alphonse Capone #85.” When it came to his many victims (at least 300 are thought to have died in the gang wars he instigated, many at his own hand), he was heartless indeed. But when it came to his family, the mobster apparently had a soft spot.