If you look around the scenes in the various chapters you will find collectible items that are books. there are 2 books that i am aware of that are spell books. unfortunately i don't know which scenes they are in. when you find these books you will unlock the new spells they contain
Throughout the game there are "Easter eggs." Sometimes you'll find a chocolate frog card, a galleon, a book or an item. There are three spell books. One that you buy from Diagon Alley, one that you find at the Christmas feast, and another that I can't remember where I found it, but it's not that hard to find.
When you enter the spell area, you will see all three books to the left and different spells to the right. Clicking each book with revel different spells you can practice (and you seem to only be able to practice the spells that will help you in a duel). It's really that simple.
Another great thing about the Easter eggs is that whenever you find one you earn your house house points!
