I heard about a cool site from Book Moot today: The Potter Index. The Potter Index is an online, searchable index of the Harry Potter books (U.S. editions), "created by Harry Potter fans for Harry Potter fans, students and teachers". You can type in any phrase you like, and it will show you where that phrase appears in the first six Harry Potter books (as well as in Fantastic Beasts and Where to Find Them and Quidditch Through the Ages).
For instance, if you type in "windgardium leviosa" (a favorite phrase in my house, though we have yet to get it to actually work) you find three matches, two from Harry Potter and the Sorcerer's Stone (Book 1) and one from Harry Potter and the Order of the Phoenix (Book 5). It's a lot of fun! Happy Searching! -- Jen