I have seen this discussed online several times. Here is a HN thread: https://news.ycombinator.com/item?id=14192894 https://news.ycombinator.com/item?id=141928

I have seen this discussed online several times. Here is a HN thread: https://news.ycombinator.com/item?id=14192894 https://news.ycombinator.com/item?id=14192894 that indicates the text Feynman first used was Calculus for the Practical Man by J. E. Thompson. Both are quality texts and as the linked HN thread notes, can be found on archive.org.