Flatland a Romance of Many Dimensions by Edwin A Abbott 

No this book does not say that this world is flat. But it is about Dimensions. It’s basically a fantasy story about a world that is flat or 2D. It is kind of a math’s book, it was written over one hundred years ago so it may seem dated in parts, but it is thought provoking to think there could be dimensions out there but we can’t see or understand. I like to think of a metaphor about a fish in a fish bowl. We are in the inside and we can’t grasp what is on the outside as we have no concept of what it would be like, we could guess thou. 

 It is a very thin book and has only 83 pages. I wouldn’t really recommend this book for a casual read, more for somebody who maybe mathematical minded and maybe interested in physics.