* Mythology: Most of his books reimagine and weave modern stories with ancient Greek, Roman, Egyptian, Norse, and Chinese mythology.
* Adventure: His stories are fast-paced, action-packed, and full of adventure, often involving quests, battles, and perilous journeys.
* Young Adult: While some of his books appeal to adults, they are primarily geared towards a young adult audience.
Here's a breakdown of his major series by genre:
Percy Jackson & the Olympians: Mythology (Greek), Adventure, Young Adult
Heroes of Olympus: Mythology (Greek, Roman), Adventure, Young Adult
Magnus Chase and the Gods of Asgard: Mythology (Norse), Adventure, Young Adult
Trials of Apollo: Mythology (Greek), Adventure, Young Adult
Kane Chronicles: Mythology (Egyptian), Adventure, Young Adult
The Demigod Diaries: Mythology (Greek), Adventure, Young Adult (short story collection)
The Sword of Summer: Mythology (Norse), Adventure, Young Adult
The Hidden Oracle: Mythology (Greek), Adventure, Young Adult
The Tyrant's Tomb: Mythology (Greek), Adventure, Young Adult
The Burning Maze: Mythology (Greek), Adventure, Young Adult
The Tower of Nero: Mythology (Greek), Adventure, Young Adult
The Trials of Apollo: Mythology (Greek), Adventure, Young Adult
The Son of Neptune: Mythology (Roman), Adventure, Young Adult
The Mark of Athena: Mythology (Greek, Roman), Adventure, Young Adult
The House of Hades: Mythology (Greek, Roman), Adventure, Young Adult
The Blood of Olympus: Mythology (Greek, Roman), Adventure, Young Adult
He also writes mystery and thriller books, such as the "Three Investigators" series and the "Aru Shah" series.
Overall, Rick Riordan's work is a mix of fantasy, adventure, and mythology, with a strong focus on young adult fiction.