It is possible to get away with just the Principles book, but if you want to reference more cases when you get your assignments you'll need the case book (you could probably also easily get this at the library because it's not very popular since everyone usually already has a copy of it).
If you need to choose, it would be better to buy different contract books (principles) by different publishers because sometimes the other books give better/more succinct explanations than the prescribed one, and also list different cases.