I think the RPG community can learn a lot from boardgame community in this matter. Boardgame manuals have long invested in proper UX, to the point that there are standard practices for it. If anything, seeing how things are organized in a boardgame manual is a good start to learn about information hierarchy.
I think the “pdf revolution” has hurt readability in our hobby. Many of the small, early 2000s products did not bother much with layout and graphic design. As long as the file was converted to a pdf, it was good to go. Then we had OSR movement, which was almost an excuse for cramming every page full of text in single or double column. After all, if it was fine for the original texts, why should we mess with it?
I am really happy where the indie market is heading right now. The hobby has grown enough that authors can afford paying for better layout AND readers expect a good layout. I love seeing reviewers commenting or giving hints on it (e.g Questing Beast)
Long story short - How to make RPG books more readable? Follow good design guidelines. Either get a graphic designer to do it for you or learn more about design and UX for books.