At the very minimum you should probably consider purchasing:
The Player’s Handbook is going to tell you everything you need to play the game. And if your character is one of the characters listed in the Player’s Handbook, then it’ll also tell you everything you need to know about your character, including powers and how to level that character. And since you can pick up a Player’s Handbook for around $10 on Amazon these days, there’s really no reason not to have one.
- Whatever handbook your character came from.
If you’re playing a class that’s not detailed in the Player’s Handbook, it might be worth your while to pick up whatever handbook it came from. See your friendly DM if you need help determining this.