It’s interesting to me that whilst Gary Gygax gets ample credit for his custodianship of 1E AD&D, Dave “Zeb” Cook isn’t similarly celebrated by 2E fans – despite the fact that Cook was arguably the game’s “show-runner” in the early 2E period much as Gary was for the early period of the game’s existence and Mike Mearls seems to have become for 5E. As well as writing the 2E core books, Cook was also the primary author of Oriental Adventures (despite Gary being given the credit), which as well as being one of the more beloved of the post-Unearthed Arcana 1E hardbacks was also the book which introduced the idea of nonweapon proficiencies to the game – a system feature which would underpin a bunch of other distinctively 2E mechanics, like the “kits” offered in the line of brown splatbooks (ew) that acted like a fiddly, class-specific, not-really-very-balanced set of forerunners to 5E Backgrounds. Moreover, between the release of the 2E core and his departure from TSR in 1994, Cook helmed two out of the three major hardback additions to the system – the Tome of Magic and the Book of Artifacts. (Legends & Lore was penned by Jim Ward and Troy Denning, building on Ward and Rob Kuntz’ previous work on Deities & Demigods).
His last major contribution to the game was Planescape. In the 1E era Jeff Grubb had produced the Manual of the Planes, taking the Great Wheel cosmology as outline by Gary in previous works (notably the 1E Dungeon Master’s Guide) and stacking a whole bunch of dry rules detail on it. Interesting in principle, it was felt that it didn’t really support much in the way of adventure on the planes, and when 2E rolled around the idea started brewing of giving it an update with an eye to using the planes as a basis for campaigning in their own right.