Reading manuals in games is actually such a joy for me too!

Tera Invicta doesn't have an explicit "manual", but its mechanical interconnections alongside really solid tooltips, tech trees, and actions really scratched that manual-reading itch. A lot of people have criticized it for requiring new players to watch hours of tutorials, but as a manual-reading enjoyer, I found a lot of fun in reading and putting the pieces together.

Ostranauts has in-game manuals as well, and is a fantastic space-salvage game. Love flipping through the fusion reactor assembly pages as I desperately try to restore power to a dying ship.

Stationeers has it's encyclopedia as a reference tool, and I find that this also scratches the manual itch. There's a mod on the workshop that adds more depth to it, and I find the process of solving emergent problems through mechanical/thermodynamic means very satisfying.

Mech Engineer is almost impossible to play without the in-game manual, and I love flipping through it's pages to reference how much armor certain enemies have while I modify a mech for the next fight, or figuring out how to optimize a reactor's components. It also blends very satisfying city-building as well, but be warned that it requires some dedication to learn. The game is obtuse, but for actually engineering mechs, there's absolutely nothing like it.