Tim O'Brien became a father at 58. His two boys, Timmy and Tad, are now in their teenage years, and over the course of their life, O'Brien has written occasional letters to them: advice from an aging father about things they may take with them through life.

O'Brien, author of many works, including the collection of short stories from the war in Vietnam, "The Things They Carried," collected those letters in his new book, "Dad's Maybe Book." He joined Boston Public Radio on Monday to discuss it.

The title, he said, comes not only of the maybes of his time in Vietnam, but also from getting old.

"It is lifelong. It goes back at least as far as Vietnam, when every step was a maybe step. I served in [the] Quảng Ninh province, the most heavily mined area in Vietnam. ... Every step I'd look at the earth and think, 'Is this it? Is this it?' and that became infectious for me. I carried it home with me from Vietnam. But that maybeness remained with me in all kinds of forms for the rest of my life, and returned with a vengeance when I got old," said O'Brien. "We're all writing our maybe books, full of maybe tomorrows. ... It's not just specific to me. I think it's all of us."

The book is an attempt for O'Brien to share himself with his sons through vignettes he assumed they would only read after he died, he said, but it is also a meditation on aging and parenthood.

"I had defined myself, my sense of self esteem and self love, only by the books I had written," he said. "That changed with the birth of my first son. Something became so much more important, these two precious little boys, I had to be a good father. I devoted myself to it for all those years. ... It rescued my life, in a way. It gave me myself back."