The most minimalist version would include the Player's Handbook, the Dungeon Master's Guide, the Monster Manual, and the Deities and Demigods cyclopedia. That would clock in at 622 pages (311 sheets) and seems to be a solid choice. Very old-school indeed.
However, there's a whole slew of creatures I like from both the Fiend Folio and even (gasp!) the Monster Manual II: Aboleth, Death Knight, Demilich, Githyanki, Graz'zt, Hook Horror, Drider, Gibbering Mouther, Tween, etc. Fiend Folio actually seems "old school" enough but Monster Manual II with the newer layout doesn't fit quite as nicely. Still, based on content it should probably go in. So now we're at 908 pages (454 sheets) and things are starting to get huge.
Which brings us to the really iffy choices.
I dislike Unearthed Arcana a lot. Not just because it turned AD&D into a zoo of over-powered races and classes, not just because there were a lot of "bugs" as it were, but also because plenty of it seems "ripped off" from other people's work without much in terms of careful editing. There are, however, a good number of spells and magic items that I like, and there's weapon specialization which at least gives fighters a little edge to distinguish themselves from rangers and paladins.
Then there is Oriental Adventures, a tome I actually sort of like. It finally puts monks where they belong and it does provide some fascinating options (such as the shukenja/sohei tradeoff which I have chosen to read back into the original game as a priest/cleric tradeoff). But it's also the first 1e hardcover that is obviously not Gary's work anymore. At least for Unearthed Arcana you can still show his involvement (witness the trail of articles he wrote in Dragon magazine) even if the source material was "collected" from all over the place and therefore lacks a unifying vision. But Oriental Adventures?
In any case, if I added those two volumes as well we'd be at 1180 pages (590 sheets) which is probably as much as any sane book binder would put together, not to even speak of how much that thing would weigh. So paraphrasing Gary a little:
What would you, Gentle Reader, put into your "ultimate 1e" leather-bound tome of eternity?And yes, I do feel bad about slicing up perfectly good hardcover books, but I really wouldn't want to use scans or copies or whatnot. I have a secondary plan which involves carefully separating and (where appropriate) framing the covers that are still in a good enough state for that treatment. Those that are "too worn" already I'll probably turn into my "ultimate 1e" DM screen instead. So hopefully that makes up for the "sin" of tearing them apart in the first place?