His dad found a supplier who was flush with recycled library books. So the Simons bought a lot of them just before Christmas 1974.
"The fly in the ointment was that we moved them on a guaranteed-sell basis," Mark Simon said. "They didn't sell. They started coming back to us after Christmas by the freight-car load."
So many books were returned they had to rent a warehouse to store them. They were stuck.
Simon married and left for a two-week honeymoon in June 1975. When he got back, a message awaited him: Meet your father the next day at your bookstore in Atlantic City.
Marty Simon had been walking along the forlorn Boardwalk - this was before casinos - and he mentioned to a lady friend how he should open a store, something he used to say a lot. She called his bluff.
When Mark Simon saw Atlantic Books for the first time, the place was packed with customers. "Everyone on the Boardwalk was carrying bags of books," he said.
The price was right: What didn't cost $1.25 went for $1.50. They were popular hardbacks that libraries lend and then get rid of when new titles make the best-seller lists.
Atlantic Books started opening up and down the Shore, catering to the masses who fill vacation homes and motels for a week then make way for new guests.
"They have time, they have money, and the beds change," he said. "My father figured you either changed the product or you change the customer."
After they ran out of resort towns, they started opening stores in the cities, starting with Pittsburgh, then Philadelphia around 1980, when they took a space at 1208 Chestnut St. By then, they were selling magazines and new books, too.
Business was good in the cities initially, then not so good, he said. After his father fell and broke a hip in 1994, Mark Simon wanted to find an investor who would help him become sole owner of the business. When that didn't work, he sold the place to Deb Shops, a juvenile-fashion chain.
It hired him to run the business for five years, but by 2001, it had lost interest, as bookstores were starting to feel the bite of online retailers. To keep the company from shuttering all the stores, throwing hundreds of people out of work, Mark Simon bought Atlantic Books back.
He got a good deal, he said, but each year has been more difficult. Five years ago, he told his partner, Ross Laufgraben, they'd never survive the e-readers. He was right. In summer 2010, they decided the only way to avoid bankruptcy was to convert their inventory to cash - "go out of business like men," he said.
So last summer they closed seven stores. The four left - in the Montgomeryville Mall; Stafford, N.J.; and Dover and Rehoboth Beach, Del. - will stay open only until the shelves are empty.
Talking about his customers and his longtime employees made Mark Simon's eyes begin to mist. Over coffee and sesame bagel at the 401 Diner in Conshohocken, he talked about his next move.
"I'm 62. I've never been unemployed. I've worked one way or another since I was 14. Unfortunately, I can't collect unemployment. My wife and I are going to go sailing for a while. If something comes my way, then I'll figure out what I want to do when I grow up."
Contact columnist Daniel Rubin at 215-854-5917, firstname.lastname@example.org,
or @danielrubin on Twitter. Read his blog at philly.com/blinq.