Honestly, I think RPG books could definitely use some deconstruction into the design goals.
Off the top of my head in no particular order:
I tend to just skip over fiction pieces altogether. Especially the multi page stories commonly found in World of Darkness games
Now I have to write a Cthulhu Dark based epic fantasy game.
It would actually work really well, I think. Cthulhu Dark is already pretty similar to the corruption mechanics in Polaris, so the game could be about pure heroes slowly become corrupted or wearied of the fight, and trying to win before the corruption overtakes their souls.
One first step is to jettison everything that assumes the user might not know what an RPG is, how it works, or why you’d play one. Not just in the intro, but throughout the text. Cookbooks don’t explain what cooking is and video games don’t have a ‘what is an X-box’ tutorial.
I read the Lamentations of the Flame Princess Rules & Magic corebook recently and it did this to an aggressive degree. It was an interesting choice, and did make the book more readable to someone like me who knows what’s what.
But I’ve also read a lot of 70’s and early 80’s rpgs lately and they’re often downright baffling because they don’t explain the context of play. The nature of roleplaying has changed enough that I find it hard to divine their base assumptions.
One first step is to jettison everything that assumes the user might not know what an RPG is, how it works, or why you’d play one. Not just in the intro, but throughout the text. Cookbooks don’t explain what cooking is and video games don’t have a ‘what is an X-box’ tutorial.
I disagree with this. It assumes that all role-playing games are played in roughly or even specifically the same fashion and that’s just not true. There’s no interesting cost in adding a short section on exactly what play should look like and what the expectations are for each player role.
I’ll be your Huckleberry if you want a sounding board/play tester 
Back on topic, I don’t read much of the fiction either, except for play examples if a mechanic is unclear. And then, as often as not I’ll look up an actual play video before I run the game.
I’m rather relieved, at least, that we’re moving away from “It’s like cops and robbers but we have rules so that nobody brings a magic force field and ruins our fun”.
This is something of a sidetrack from readability, but when I read those “what is roleplaying” notes, they often feel like an afterthought someone has put in because all rpg books are supposed to have them, without really thinking about it.
I mean, you could make an analysis: Is this book going to be an entry point into roleplaying? If its something like D&D or Vampire, the answer is probably yes. For other types games, it could be no.
The most half-assed “what is roleplaying” I’ve read recently was in Symbaroum (otherwise a very good game), where they used jargon like PC without explaining what it meant. I mean, if someone knows what PC means, they don’t need the “what is roleplaying” intro.
Sometimes I think these are a lost opportunity: Instead of defining what is roleplaying, that same energy could be used to define roleplaying in the specific context of this game. After all, D&D roleplaying is quite different from A Quiet Year roleplaying.
I like to have a concise introduction chapter that explains, a) what the game is about & what stories you might tell with it, and b) how the book itself is structured. Even if I’ll never read it again, it would help me that very first time.
Also: Consider using sidebars. Not only are they great to condense information, give explanations or tips, or provide a bit of fluff, they also serve to break up the text a bit so it doesn’t become a wall.
How do people feel about print book dimensions? On one hand I love having nice big art pieces in Spire and Tales from the Loop. On the other, I generally find smaller form books like Apocalypse World, Monsterhearts, Fiasco, and Blades in the Dark much easier to read.
I really wish more publishers would release Kindle or epub editions of their books. Yes, art and graphic design can convey a lot about the tone and fluff. But reading a multi-hundred page PDF tends to be annoying no matter how well designed or bookmarked.
I’m personally experimenting with, and enjoying, designing in booklet format for my projects. Couldn’t tell you how it compares but I know I personally prefer a smaller form factor.
For pure readability, I’ve come to prefer the slightly smaller size Blades in the Dark and the Fate books are in. It’s less exhausting because there’s less stuff per spread, but there’s still space for visuals.
Both of the game’s I’ve done on my own are in somewhat similar page sizes and I think I’ll stick with it in the future as well.
Smaller form factor is vastly preferred by me. Way more portable, easier to leaf through.
I agree: more epubs. They outline better accessibility than PDFs.
Oddly enough, I prefer larger print formats and wish my favorite games used them.
I enjoy smaller, portable books when I’m reading for leisure, but if I’m trying to grok the system, reference it in play, or use it for generating characters, places, and things – even with great layout – I have to weigh down the pages to keep it from closing, and then I have to turn the page up to five or more times to see something that could have been laid out on a 8.5 x 11 page.
I think the real problem is that traditional publishers, like Wizards, use large print formats but don’t use the extra space to their advantage. While design savvy indie publishers use smaller formats out of tradition and partly because of cost.
In other words, to get back to the thread question: layout, writing, and marrying the two.
Its interesting to hear about how differently we read rpg books! Personally, I find boardgame style dense rules text hard to read, so I prefer a loose style, with fiction elements to give it more air. I find it sad that the use of fiction text in rpg books gets such a bad rap, although I’ve certainly read my share of books where such material is low quality and pointless.
I think White Wolf did a lot to contribute to that, with pages of pure fiction you turned past, searching for where the rules began. Not to mention that it was always nigh impossible to read even for someone with good eyesight, in the name of looking goth or edgy or something.
Yeah, I know what you mean. In the interest of full disclosure, I got to confess that I wrote a lot of that stuff for V5, especially for the Anarch book. But I like to flatter myself by thinking that I wrote entertainingly…
Edit: During that work, I read a ton of old WoD books, so you might guess where this interest in readability comes from 
For me, the problem is less the quality of the fiction, more the big block of text I’m wading past to get to the game itself and understand what it’s like. For me, fiction matters less because you can build a good story around any game if you try hard enough. (We joke about the whole “d&d can do anything” idea, but there’s people who really can make it do anything!) I’m more interested in understanding how the game engages and promotes particular fiction. So, having swathes of fiction in an RPG book doesn’t really help me.
It also didn’t help that I was reading via PDF, honestly. With a PDF, the less scrolling I have to do to read a particular bit, the better, so any pages I scroll past are a waste of my attention and energy.