To start, the ROP- Report on Proposals, ROC- Report on Comments, and the NEC for each cycle
The NEC is revised every three years (code cycle), the ROP,& ROC and the NEC are published each code cycle.
Each ROP & ROC is the size of a NYC phonebook, you could probably go back about 15 code cycles for a good start. The books, if piled one on top of the other would probably stand to the ceiling of an average room.
GOOD LUCK, HAVE AT IT!!!
P.S. That actually might be fun if you are interested in the history of the Code