The ending is pretty open since we don't know if Waverly keeps playing chess and if her relationship with her mother will remain broken. She even turns her mom into a chess opponent straight out of a Stephen King novel:
Opposite me was my opponent, two angry black slits. […] My white pieces screamed as they scurried and fell off the board one by one. (67-68)
Waverly's imagined opponent (a.k.a. her mom) thrashes her badly, and we're left to wonder if either Waverly or her mother will ever recover from the break. Importantly, the argument with her mother seems to set Waverly free:
I felt myself growing light. I rose up into the air and flew out the window. (68)
No more pressure to win, no more mom hovering around her dropping snarky comments about doing better. Sure, Waverly's all by herself—"everything below me disappeared and I was alone" 68)—but she's still so young, and her lonesomeness seems to suggest that she can now make her own choices instead of having them made for her.
As for not knowing what happens next, well, "Rules of the Game" is part of a much longer book… so just keep reading.