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The Leaky Cauldron
An incredibly detailed site concerning all things Harry Potter. Hailed by J.K. Rowling, this site contains answers to all of your Harry Potter questions.
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The Harry Potter Lexicon
An amazing collection of information, background, and minutiae having to do with the Harry Potter series, including maps and a timeline. There's also an essay section.
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Math Stories: Harry Potter Theme
Do you like to read books but hate doing your math homework? Check out this site to find out how to integrate math skills into your novel. Cross-curricular activity sheet.