The Photoshop 2.5 manual (1992) is a thing of beauty. It is like an introductory course in digital imaging, well structured, put together with care and expertise, it provided to me a fascinating introduction to (at the time for me) mind-blowing concepts in digital artwork. It explained the fundamental concepts in digital imaging that have remained with me ever since.

I concur. Old docs are the good stuff: https://passo.uno/why-collect-read-old-computer-manuals/