For those who might not have heard about it in their newsfeeds yet, there is a 8 book megabundle sale on the AD&D Core Book PDF's. Currently only $14.95 vs the usual $75.
Here are the books that you get in the bundle:
Dungeon Master's Guide [Premium Edition]
Dungeoneer's Survival Guide
Monster Manual [Premium Edition]
Monster Manual II
Player's Handbook [Premium Edition]
Wilderness Survival Guide
FYI if you've already bought some of these titles previously, you'll get a discount for that title.
Now, for me, the Survival Guides are not Core Rules but that's just personal preference. Also the Deities and Demigods is missing from this which I do consider core rules. YMMV.
Still a great bargain if you don't have some or all of these yet. Plus there's the BECMI Known World bundle as well if that's your cup of tea. Plus a whole bunch of other things worth checking out.