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The Lightning Thief
After learning that he is the son of a mortal woman and Poseidon, god of the sea, twelve-year-old Percy is sent to a summer camp for demigods like himself, and joins his new friends on a quest to prevent a war between the gods.
Why it's similar
Both follow an ordinary boy who discovers a hidden supernatural heritage and is sent to a specialized training sanctuary alongside two loyal friends. Both stories blend ancient mythological or magical elements into a hidden contemporary world.
What's different
Riordan uses a snappy, modern first-person voice set across the United States, whereas Rowling adopts an episodic, British boarding-school mystery framed in the third person.