According to the official Adobe website, "The rule of thirds is a composition guideline that places your subject in the left or right third of an image, leaving the other two thirds more open." You may have heard or seen the rule of thirds from your phone camera settings where it is generally known as the guide lines. The guide lines itself is not the rule of thirds, but it does help a lot. I mainly use the rule for my filmmaking and photography, but it could also be used for everything like book designing. Even if the contents of the book are really good if the execution and the way it is shown in the book is bad, it will most likely be a pain for the eyes and ruin the overall reading experience.