Folks like roleplaying games to read like fiction, like technical manuals, or like something in between. The in-between options are generally the best received. In my experience, people who argue strongly for technical-manual style works don’t necessarily like games written this way, but feel the need to argue for them.
Where they are right, though, is that you need to be mindful that between looking up references, taking deep dives into lore when inspiration strikes, and all the other reasons we crack open games, they need to be friendly to non-linear reading. Here’s how you do that.
Use Headers Well
In the past, header hierarchies were mostly a concern for indexers and for sight reading. This is no longer true, because PDFs and other electronic reading formats do a hell of a lot with headers. You’ll have a descending 1 to 4 hierarchy (4 is the limit for most publishers, and I try not to go lower than 3) through a template or as tags such as <h1>for the top-level header of a chapter. You should have everything under an H1 be related in some way until the next H1. H2s cover a subtopic of H1 content, and down the line.
Your headers should break up big walls of text so the reader can zero in on relevant sections. A two-page spread that isn’t broken down by heading and subheading is often undesirable, though I’m going to talk about exceptions below.
It’s much easier to find relevant text when it’s organized into short, punchy paragraphs. Readers scan for words and phrases of interest, and this makes them easier to find.
Okay, Go Long
There are in fact times where you can, ans should, do longer blocks of text, from big paragraphs to the spreads I told you to avoid up above. This is when your intention is to provoke immersion into the text. This is usually limited to fiction and setting description, though a number of weird exceptions exist. In essence, if you want someone to read it like a short story or encyclopedia entry, go long. For example, Swords and Glory Vol. 1, which is a deep dive into the world of Tekumel with no game mechanics, has very long chunks of text, and it works, because you need time to settle into the Five Empires, and because Tekumel is so densely packed with challenges to your assumptions it needs the space.
Get Boxy or Go to the Bar
Text boxes and sidebars pop out at the reader without interrupting the rest of your work. If you have a high level summary of an adjacent section, put it in a box. Boxes and sidebars can also be used to cover a topic from an alternate perspective. In a lot of my World of Darkness work, which followed the convention of writing with an in-universe slant, sidebars gave me a chance to clarify things from an almost-objective point of view. If you’re writing about how the Empire is well-guided by a divine emperor, a sidebar is often the place to note that the whole thing is propaganda to hide the Imperial Guard’s management of an idiot in a fancy hat.
Be aware that while I am using sidebar and text box interchangeably, they are not necessarily interchangeable to the person doing layout. Follow whatever rules are in force.
What is the Reader Doing?
All my advice hinges on that question. There are multiple ways to read the various chapters and chunks of a book. Identify how you wish something to be read, and tune your text and organization accordingly.