In an interview for the Yorkshire Television program Take the World from Another Point of View (1972), Richard Feynman said:
You see, I have had in my life a number of pleasant experiences. One of the earliest ones was when I was a kid I invented a problem for myself, the sum of the powers of the integers, and in trying to get the formula for it I developed a certain set of numbers, the formula for which I couldn’t get, and I discovered later that those were known as the Bernoulli numbers and discovered in 1739. So I was up to 1739 when I was about 14 you see. And then a little later I discovered something that I’d find out I just may have invented a thing which we now call operator calculus. That was invented in 1890-something. Gradually I was inventing things that came later and later.
But the moment when I began to realize that I was now working on something new was when I read about quantum electrodynamics at the time and I read a book, and I learned about it. For example, I read Dirac’s book, and he had these problems that nobody knew how to solve that were described there. I couldn’t understand the book very well because I really wasn’t up to it. But there in the last paragraph at the end of the book it said, “Some new ideas are here needed.” And so there I was. Some new ideas were needed? OK! So I started to think of new ideas.
What book does Richard Feynman refer to exactly?