What is your preferred software for assembling rulebooks, supplements, etc?

Open Office Writer is all I need for 1-7 pages.

I've used Scribus for almost everything so far. I planned to migrate to Affinity Publisher and got myself a copy on sale right before the Pandemic hit. But then it did hit, and I got super depressed, and haven't really published anything since, aside from fulfilling the in-progress Kickstarter at the time, which was already grandfathered into Scribus. So I guess it's still technically Scribus. I used to use Inkscape for the character sheets though. And the cover art, usually. If it's small enough, sometimes I just use LibreOffice or even just the PDF export functionality baked into Google Docs, but if it's more than like three pages it's always been Scribus.

Scribus is very easy, specially if you make a Master Page (that are page templates). Then you can re-use the one you did as a template for your next tutorials. Finally you export as PDF.

You can configure a text box with the font, color and everything, and just copy and paste it for each paragraph.

Scribus is so intuitive that you can just use it directly without watching tutorials or reading anything, at least for me. Everything is in the correct menu, and everything is simple.

Trick: If you want a colored background you can edit your Master Page, make a box as big as your page and fill it with color, then lock it.  You can add the page numbers and any header or footer to that Master Page, and just make pages based on that one.

I tried using Scribus a few years ago. I found Affinity Publisher is way more intuitive. I’ve been using it since 2019. I’ve since bought the whole suite, mostly out of how they integrate with Publisher.

Affinity Publisher. I've tried Scribus, but after 3-4 hours I've created a Cheat Sheet for my first game, which was as mess as I would assemble it in 10 minutes in Paint instead.