We should also give credit to Ted Peterson, who wrote the Pocket Guides to Empire and a lot more. I hope MK and Ted Peterson were hired to write for "Skyrim" also, but I haven't heard anything.
Did Tedders do the first PGE? I was always under the impression that was MK for some reason. I suppose I'm basing that on absolutely no evidence, though.
Anyway, as far as the series goes, I think it was Ted Pererson and Julian LeFay who created the initial TES world up through Daggerfall. The intial locations and what not were from a DnD campaign they ran in their spare time between making bowling games. Tedders was probably the driving force behind Daggerfall and a lot of the lore that comes from that era. MK came on board during Redguard and was enormously influential in the direction the series took for Morrowind. MK is best known for (and best at) the "weird stuff" but I'm pretty sure he's done some shockingly normal books as well. I was a lore expert back in the Daggerfall days, but haven't been able to keep up as much I would like. There are certainly people in these forums who can tell you who wrote what book, when, why, and what was on the radio when they wrote it, so I imagine they will chime in eventually.