NEW YORK (AP) — Starting this fall, fans of Rick Riordan’s “Percy Jackson” books can discover some adventures that took place at the demigod training facility Camp Half-Blood when Percy himself wasn’t around.

Random House Children’s Books announced Tuesday that Riordan is collaborating with different co-authors on each of four “Camp Half-Blood” middle-grade novels set during the time between Riordan’s first two Percy books, “The Lightning Thief” and “The Sea of Monsters.”

Since “The Lightning Thief” was published in 2005, Riordan’s “Percy Jackson” novels have sold more than 150 million copies worldwide and have been translated into dozens of languages. His books feature such beloved characters as Percy, the boy whose father is the Greek god Poseidon; best friend Grover Underwood and Annabeth Chase, daughter of Athena. Riordan said in a statement that working on the Emmy-winning Disney+ adaptation of his work stirred his curiosity about Camp Half-Blood.

“It made me wonder what was happening back at camp between the books of the original series, when Percy, Annabeth, and Grover weren’t around,” he said. “This new series gives me a chance to explore that idea, while opening up Percy’s world by introducing a new group of demigod heroes, co-created with amazing authors who can write authentically about those heroes’ backgrounds. It’s been an incredible thrill!”

The first installment, “The Wild Zone,” was written with Annebelle Oh and comes out Sept. 29. Kyle Lukoff, Jade Adia and Pablo Cartaya are working with Riordan on the second, third and fourth books, respectively. Each will be published six months apart.

According to Random House, “The Wild Zone” tells of “ancient threats and long-buried secrets” and how “campers begin to disappear, unlikely new heroes must face monsters, curses, and impossible choices, all while discovering what it truly means to belong.”