“The kids,” Mr. Bradfield recalled, “looked at me as if I’d sprouted a second head, and were like: ‘Yeah, duh. Of course!’ ”

The day camp, which is held in a state park, attracts children from as far away as Brazil and Britain, who stay with their parents in nearby hotels. This year, the camp’s 450 spots sold out in an hour and a half, Mr. Bradfield said.

The camps run by bookstores, which are also in Decatur, Ga., and now in Brooklyn, are not fancy affairs. A casual observer of the various Camp Half-Bloods would see a few decorations and children in matching camp T-shirts jousting with foam swords or javelins. Gods, oracles and monsters are played by actors, counselors or volunteers.

But the homemade nature of the experience, camp staff members said, permits students to create the illusion in their own minds.

“My biggest challenge has been getting parents to understand that we don’t intend to sit indoors with their kids and read all summer,” said Crystal Bobb-Semple, the owner of Brownstone Books in Bedford-Stuyvesant, which started the Brooklyn camp this summer and charges $375 per week. “It is experiential.”

Camp Half-Blood in Brooklyn had 44 campers and a simple story line. Discord had come over Brooklyn. It was up to the campers, as demigods, to find the five pieces of the Apple of Discord, a mythological object, to set things right.

Not everything went according to plan. Prospect Park denied permission to put up a tent, so the camp’s center consisted of three folding tables under tall trees. A nearby library and a movie theater were used in bad weather.