(As I said in a previous post, Buffett: Making of an American Capitalist is the best biography on the great investor.)
That said, The Snowball is not entirely without value. Here is Schroeder at her best, explaining derivative contracts (on pages 544 and 545):
Derivative contracts work like this: In the Rockwood Chocolate deal, the value of hte futures contract was "derived from" the price of cocoa beans on a certain date. If the beans turned out to be worth less than the price agreed to by the contract, the person who had bought the futures contract as insurance "won." Her losses were covered. If the beans were worth more, the person who had sold the futures contract as insurance "won." The contract entitled him to buy below the then-current market price.
Suppose that in the weight deal Buffett had made with Howie for the rent on his farm, he didn't want to risk Howie's actually losing weight, which would drop the rent. Since this was under Howie's control, Warren might want to buy insurance from someone else.
He could say to Susie, "Lookit, I'll pay you a hundred bucks today. If Howie loses twenty pounds and keeps it off for the next six months, you'll pay me the two thousand dollars of rent that I'll lose. If he doesn't keep it off for the whole six months, you don't have to pay me the rent and you get to keep the hundred bucks."
The index that determined the gain or loss was "derived" from Howie's weight, and whether or not Buffett would make such a deal was based on a handicap of the odds that Howie would be able to lose the weight and keep it off.
Anather example: Suppose that Warren made a deal with Astrid to give up eating potato chips for a year. If he ate a potato chip he had to pay her a thousand bucks. This would not be a derivative contract. Warren and Astrid were simply making a deal. Whether Warren ate a potato chip was not "derived from" anything. It was under his control.
However, if Astrid and Warren made that agreement and then Astrid paid Warren's sister Bertie a hundred bucks as insurance, in exchange for a thousand dollars if Astrid lost the bet, the deal with Bertie would be a derivative contract. It would be "derived from" whether Warren ate the potato chips, which was not under either Bertie's or Astrid's control. Astrid stood to lose the hundred bucks to Bertie if Warren didn't eat the chips, and Bertie would lose a thousand bucks if he did.