Main Series:
* Percy Jackson & the Olympians: This is his most famous series, based on Greek mythology. It follows the adventures of Percy Jackson, a demigod, and his friends.
* *Percy Jackson & the Lightning Thief*
* *Percy Jackson & the Sea of Monsters*
* *Percy Jackson & the Titan's Curse*
* *Percy Jackson & the Battle of the Labyrinth*
* *Percy Jackson & the Last Olympian*
* Heroes of Olympus: This series continues the story after *Percy Jackson & the Last Olympian*, focusing on a new group of demigods facing a new threat.
* *The Lost Hero*
* *The Son of Neptune*
* *The Mark of Athena*
* *The House of Hades*
* *The Blood of Olympus*
* Magnus Chase and the Gods of Asgard: This series features Norse mythology and follows the story of Magnus Chase, a homeless teenager who discovers he is the son of a Norse god.
* *The Sword of Summer*
* *The Hammer of Thor*
* *The Ship of the Dead*
* Trials of Apollo: This series follows Apollo, the Greek god of music and prophecy, who has been stripped of his divine powers and is now a mortal teenager.
* *The Hidden Oracle*
* *The Dark Prophecy*
* *The Burning Maze*
* *The Tyrant's Tomb*
* *The Tower of Nero*
* Kane Chronicles: This Egyptian mythology-based series follows the adventures of Carter and Sadie Kane, siblings who discover their magical heritage and have to prevent the Egyptian god Set from destroying the world.
* *The Red Pyramid*
* *The Throne of Fire*
* *The Serpent's Shadow*
Other Books:
* The Demigod Diaries: A series of short stories about various characters from the *Percy Jackson & the Olympians* and *Heroes of Olympus* series.
* Percy Jackson's Greek Gods: A retelling of Greek mythology, focusing on the gods and goddesses.
* Percy Jackson's Greek Heroes: A retelling of Greek myths focusing on heroes.
* The Demigod Files: A collection of information on demigods, monsters, and other elements from the Percy Jackson universe.
* The 39 Clues: Riordan wrote several books in this series, which follows a family on a quest for hidden clues related to their heritage.
Note: Riordan also has several books for younger readers, as well as books written under the pen name "Rick Riordan."
This list is not exhaustive, and Riordan continues to write new books and series! You can find more information about his work on his website or on Goodreads.