A book on a 2 meter high self has 20 joules of potential energy. what is the mass of this book?

As this is a simple plug and chug, you clearly have not learned the physics. Start with the definitions as a first step in learning physics. In this case, what is the definition of PE from the work done against the force of gravity?

The work done against the force of gravity, W, is force X distance moved. In math talk, that's Q = FS = Wh where W is the force of gravity, aka, weight, and S = h is the distance the weight is moved, in this case lifted to a height h.

So the physics is this. You store energy, which is called potential energy, when doing work on that book by lifting it to a height h = 2 m. And given is that Q = 20 J = Wh = W*2 so the weight of the book is W = Q/h = 20/2 = 10 Newtons.

And when near Earth's surface, we know that W = Mg where M is the mass you're looking for and g = 9.8 N/kg is the gravity field strength. So M = W/g = 10/9.8 ~ 1 kg. The mass is a tad over 1 kg. ANS