Old game manuals weren't always just dry little pamphlets of backstory that couldn't fit on the cartridge1, but sometimes were complete works of art in their own right.
The original Fallout 1 instruction manual was designed as a Vault-Tec survival guide, "pinned" with notes throughout from the overseer often covering up sections with their own commentary about the mission you were being prepped to go on. Only when it got to the bits on installing and running the game did it break character, if I recall correctly.
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That even these manuals are frequently not available with the official re-releases is really unfortunate, in my opinion. Try playing old Phantasy Star games without prior knowledge or a guide on what the technique names mean.
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The Starsiege "compendium" a big in-fiction lore book that was included in the box of Starsiege (1998) for PC. It also had a ton of comic art and now-crude CG concepts and the peak of late-90s graphic design passion. A classic of videos game toilet reading
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The Homeworld Manual's Historical-Technical Briefing - Written almost entirely diegetically, it had a minimum of character art, but a ton of abstract symbology and detailed descriptions of Kharak society and history before the discovery of the hyperspace core. The Homeworld world was so built-up in my head as this exotic alien society that I was super bummed when Homeworld 2 came out and i realized that all the Hiigaarans were just human-lookin' people.
Also shout-outs to Wing Commander and its downloadable novella. Even the first Baldur's Gate manual had little notes and sections allegedly written by Volo (with comments from Elminster in the margins).
Thankfully both of these are available for free download from the Sierra Chest fansite in PDF format (the Starsiege Pilot guide, which has fiction, but is less lavish for needing to teach you how to play video game, happens to be on Archive.org).
