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I haven't read many game manuals, because I didn't have access to original games back when game manuals were still a thing. But the manual for Master of Orion was available for free online at one point, and I grabbed a copy. I like it because it does a great job at making all the game mechanisms explicit and transparent, while also adding flavor... and I still wasn't able to beat the AI even once. That's how good the game is.
No, seriously, that particular manual is a shining example of good organization and clear writing, while also being entertaining to read. If I ever make a game big and complex enough to warrant a manual, the one for MOO is the standard I'll hold myself to.