Ian didn't kill himself. He was killed by mook thugs hired by Irene's father (who thought he was her boyfriend). Still, he'd pretty much given up on living anyway after thinking the woman he'd met three years ago had died. If Jim's novel is truly a memoir, it's Jim himself who ends up committing suicide a year after it's published. Considering the last page has Jim looking homeless and carrying Ian's hat, it's certainly possible that's the decision he made.