Wandering through an antique mall over the weekend, I came across a 1947 NEC Handbook, in good condition, priced at $9.00. I declined to buy it, but it was interesting to see what the dealer thought that bit of history was worth.
I buy every pre-WWII electrical book I can get my hands on if the price is right. There are only 50 NEC editions however some of the early ones can sell for well over $1K. I've have about 10 editions at this point in time.