The Percy Jackson books heavily re-tell/paraphrase/rely on Ancient Greek myths.
If one makes an assumption that it's best to read the original myth before the Percy Jackson text using it, what is the minimal set of mythology that needs to be read before the 5 books of Percy Jackson and the Olympians?
E.g., you clearly must read a whole of Odyssey (especially for book #2), Hercules myths (for book #3, at least), the story of Perseus (Percy's namesake, and Medusa is mentioned in Book #1), and the story of Theseus for book #4.
But do you need any of the Iliad? Aesop? Oresteia?