It includes all three original rulebooks and all four supplements. Each booklet has new cover art, but is “otherwise a faithful reproduction of the original, including original cover art”.
As you can see, the mock-up promotional picture shows eight booklets. I’m guessing that’s an error. OTOH, if WotC had somehow convinced Rob Kuntz to finish Supplement V: Kalibruhn (the original unfinished manuscript of which Noble Knight auctioned off a few years back) so that it could be included in this thread, I’d be spending the next nine months drooling. (Or maybe they’ll include either Chainmail or the Swords & Spells supplement, although neither is listed among the current contents.)