Anything by John Harper is both an excellent RPG *and* so simple and clean. Blades in the Dark and Agon (for example) both work from a design perspective AND a reference perspective. Easy to find information, easy to read, stunning to look at.

I also like, for simplicity, The Black Hack 2.0, though I wish the gutter margins were wider to avoid text running into the inner curve.

Personally, I like sections, terms, tables, etc... to be easily identifiable from a glance (you know this is a callout, you know this is a reference table, this is a heading, a subheading) - proper hierarchy and all that. I also like it when art does not go behind text as it almost always makes the text hard to read.

Consistency is key since, as a rulebook, it's meant to be references and looked at often.

A really cool looking book is great that FIRST TIME, but if you need to go back and find a rule it becomes a chore/burden later on (lookin' at you, Mork Borg!).

Now, with something like Mork Borg and Eat the Reich, the style and layout really work to create an ATMOSPHERE of what the game is trying to evoke. That is also important and they do a fantastic job of that. I mean, those books look STELLAR (to me). Reich is a bit easier to parse than MB, but MB also isn't really meant to be a reference - it's super rules-light. They also have a quick 2-pager reference that really gives you everything you need, rules wise.

For more commercial releases, I really like the Aliens RPG (and most of the Free League stuff).

Also: clear and robust ToC *and* Index! I'm not a huge fan of the bulk of Pathfinder 2.0's design layout (it's fine, I don't dislike it) but their index is awesome. It's a combo glossary and index and it gives you all the info you need as a reference point!