In non-fiction works, the rules of your book are simply the rules of reality. In order to have a consistent quality throughout your book, I recommend deciding early on how deeply you will be researching specific topics. For the reader, it might be odd to have one chapter full of references, while the others only scratch the surface. If you indeed need different rules for different chapters, make it clear. Ideally, write down these rules in a separate document and have your editor check whether you are following them. In Agile terms, this is called Definition of Done—conditions that have to be met in order for your editor to accept your work. This document could also include things like grammar rules, spelling, rules about usage (e.g., capital letters, lower case, contractions), formatting, citation styles, image resolutions (what prints well?), caption style, tone (formal, informal), or perspective (first person, third person, etc.). This document is typically referred to as a style guide.
In fiction works, after creating your world and characters, it is time to put them into a specific situation and think about how they would act. This ensures that they will come to life and that they really are the actors of your story—and do not seem like they are hanging from the strings of a puppeteer.
If your book follows the (fictional) world’s rules throughout (with very few exceptions), you keep your readers on edge, not knowing how the characters will come out on top. Breaking those rules is possible, but should be done only sparingly to put more emphasis on a scene, for example, “Sometimes, Superman can overcome even kryptonite.”
If you do not provide limitations for your fictional characters, a common mistake is to create “Mary Sue” characters who always do the right thing. This approach means that your characters are driven by the plot, rather than driving the plot. It leaves out internal development. You should handle characters as people who have gone through a series of defining experiences. In order for them to be believable, they had to discover their own strengths and limitations. Even if you want to portray an ideal, you need to explain how they became who they are—this is true even for superheroes. You cannot simply wish yourself to be a hero. Someone who thinks that he or she can do anything without effort is not a superhero but a Mary Sue.
Mary Sue ⋅ Mary Sue is the term for a seemingly perfect fictional character. It originated from a parody of Star Trek fan fiction stories where writers included new characters with a major role in the story, but without making an effort to describe how they reached their position.
Likewise, if you allow your characters to have superpowers when it is convenient, it reduces the relevance of the actual climax of the story. The reader would assume that your characters are unlimited and could overcome any challenge by breaking the rules.
This also applies to non-fiction works if you are discussing people and their achievements: do not forget to include their failures, and how they dealt with challenges privately, in order to paint a complete picture of internal and external development.
Your readers will notice when you are trying to play God within the book’s world. It will break the “fourth wall,” destroying the illusion that these are real characters acting, and not a writer making them act. If you want your characters to reach a certain place or situation, you can create events that shake things up, but those events have to be believable based on the rules you have set up for your world. If you want the reader to think that the person he or she is reading about is a conscious being reflecting on his or her actions, you must create and follow your world’s rules.
The fourth wall ⋅ The theater stage is usually surrounded by three walls, with the fourth wall facing the audience. In this context, breaking the fourth wall is a reference to the characters becoming aware that they are being watched and directly addressing the audience.
Before you define the rules of your book, though, you should have a clear picture of the reader for whom you are writing the book. Some writers advise using 8th-grade level language, especially when it comes to complex topics. This is the same level that, for example, Harry Potter was written in, and which around 80% of Americans can read. With tools like https://readable.io, you can get an idea of the quality of each chapter. With such a feedback tool, you can also train yourself to write at a lower language level than you naturally do. For example, I write (English) at an 11th-grade level, and my books are aimed at adults and professionals, but I am thinking about writing special editions for children.
Likewise, reading is a linear activity, and you do not want the reader to put away the book because he or she stumbled over a weak part of the story. A true page-turner requires the same quality throughout the book. One of the big advantages of using the Agile approach to writing is that (at least to a certain degree) you spend roughly the same amount of time on each part of the book. This maintains a certain level of quality throughout your book. If you catch yourself getting lost in details, it is often better to just finalize open ends (and remove half-finished texts) and prepare your book for a first print. After a break, do a complete read-through. This method helps you more than spending too much time on any one part of the book.