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.