Saw a post about trpg book length that I can't reblog, but I feel like I have something useful and thoughtful to add, so here goes!
One thing that I think gets overlooked when talking about trpg game design generally is this: layout in many ways, is game design too. Kind of like how ui is game design in video games, right? Skyrim's inventory works, but it's very clunky, and is a bad game design choice for a game with so much inventory management.
When you have a long book, (anything over 50 pages I'd say? Not confident on that as a hard and fast rule) one thing that needs to be better than a short book is layout! In a short book, you don't need to scan much to find a section you kind of remember. In a big book, layout and organization play a bigger part in keeping play smooth at the table.
A couple of relevant examples:
Mork Borg is famous for its funky as shit layout, they made that choice deliberately. It's part of getting the player and facilitator immersed in the world. The starting weapons section is a full page spread! Great to look at but not ideal for reference use. They can get away with wild layout though, because all of the most used rules are also summarized on the inside front and back cover, which keeps play moving smoothly!
Wanderhome is my counterexample. That game has relatively few in the moment rules, but the choice to lay the book out as a coffee table book actually hurts it pretty badly as a play-aid. The traits for NPCs and locations are done as spreads, and are not particularly densely spaced. There isn't a quick summary table anywhere either. This means that to scan possible traits for a new NPC or location (something you can find yourself doing often) you have to thumb 20+ pages! It grinds play to a halt!! In a loosely structured game like Wanderhome, that's a real session killer!
All this to say: how quickly I can read a game is way less important to me than how scannable and reference friendly it is. That, to me, is what often kills games with large page counts: for them to work, you need really a strong editor (especially to separate technical writing and rules text from prose explanations) and a really smart layout designer.
If you are making a book and you know it's gonna be big, making single page rules references should be a top of the list priority, even before you finish the games text. Otherwise, you're going to find that people forget or miss rules that are buried in prose or hidden somewhere among your tips and tricks for running the game well.
A good game to look to for examples of good and helpful reference is definitely Masks: A New Generation. Those reference sheets are immaculate.
