When I wrote The Lightning Thief, I knew comparisons to Harry Potter would be inevitable. After all, Harry Potter is the most successful and high-profile young adult fantasy series of all time. I love Rowling's books, and I am grateful for the impact they've had on young readers and on the revitalization of children's publishing.
And so to the question: Did the Harry Potter series influence Percy Jackson? In many ways, of course it did. I was amazed at the effect Harry Potter had in my own classroom. Something about these books struck a chord with kids. The books were humorous, adventurous, fantastical, and mysterious all at the same time. My students would read The Sorcerer's Stone fourteen times before they'd pick up another book, because they said, "Nothing else is this good." When I set out to write down the Percy Jackson story, which had started as a bedtime story for my son, I was mindful of what my students had been telling me. I wanted to write a book that would appeal to them for the same reasons that Harry Potter appealed to them.
But are some elements of Percy Jackson taken from Harry Potter? I'm always amused by comments like that, because it's not that simple. My favorite comment: One adult looked at the American cover of the book, which features a picture of Cerberus, and said, "Look, a three-headed dog. Riordan obviously stole that from Fluffy in Harry Potter." Umm . . . no. Actually, that dog is several thousand years older than Fluffy.
Many other elements of Percy Jackson are similar to Harry Potter because the novels draw from the same well of folklore and mythology. Some examples:
"Harry Potter has Platform 9 3/4; Percy Jackson has the 600th floor of the Empire State Building which leads to Mount Olympus, so this idea must've been taken from Potter." The idea of a magical gateway to another land wasn't invented in the Harry Potter series. It's as old as children's literature. Anyone remember the wardrobe into Narnia? Or the phantom tollbooth? All of these stem from mythology, and the idea of a secret entrance that led into the Underworld.
"Harry Potter has two friends, a boy and a girl. Percy Jackson has Grover and Annabeth. They must be identical." Setting aside for a moment that the characters in question have very different personalities, and Grover isn't even human . . . it's a fairly common technique in folklore and children's literature for a male protagonist to have a strong female counterpart so the story appeals to both girls and boys. Ged and Tenar from the Earthsea Trilogy. Hansel and Gretel. Theseus and Ariadne. Jason and Medea. Often there is a third companion to act as a foil for the hero. Again, this is a very old paradigm. The Harry Potter series uses it to great effect, but it by no means invented the formula.
"Percy Jackson is a regular kid who discovers he is special, and the world is really full of magic. This is just like Harry Potter." Well, sure. It's also just like the young heroes in the Greek myths.
In the second Percy Jackson book, there's a scene where Annabeth summons the Grey Sisters (who in modern times drive a New York taxi) to take Percy and her to Camp Half-Blood. This will inevitably be compared to the Knight Bus in Harry Potter. It could just as easily be linked to the pumpkin coach in Cinderella, Aladin's magic carpet, or Apollo's sun chariot. The idea of a magical form of transportation has fascinated children for centuries, and appears over and over and in different guises.
So did I get some ideas from Harry Potter? In the sense that I am working with the same patterns, toying with the same mythology, doing variations on the same centuries-old themes -- yes. But Percy Jackson is not Harry Potter, nor is Harry Potter where the ideas originated. If the Percy Jackson series accomplishes nothing else, I hope it will encourage readers to go back to the Greek myths -- one of the taproots of children's fantasy -- and discover for themselves where many of our best modern novels (including the Potter books) drew inspiration.