Adventures Dark and Deep, a what-if AD&D 1E was updated to 2E using only later articles published by Gygax, is currently available as digital bundle for $0.
Just like the edition it emulates, it consists of Player's Manual, Game Master's Toolkit, and Bestiary. I ordered print version in January and haven't regretted it:
There is a lot in there and bestiary is just stunning. Table of contents could be better, but that is nitpicking. You can't go wrong with grabbing it for free.
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