Fischer left him off his list of the Ten Greatest Players, possibly in part because he had a crappy match record as WCH: +2 -3 =2. He apparently considered his title defense matches to be "training matches" for his automatically guaranteed rematch in case he lost, although by 1963 the automatic rematch clause had been rescinded.
I've read his Best Games 1931-46 and enjoyed them very much, although I'll be damned if I can think of anything I learned from them. He was an outstanding endgame player, just like every other modern champ with the possible exception of Tal so I guess you could do worse than study his endgames.
https://en.wikipedia.org/wiki/Mikhail_Botvinnik
Who was Mikhail Botvinnik? What was his style? What can we learn from studying his games? What made him special or stand out from the other world champions?