And of course Mr. Feynman Goes To Washington . I think the ultimate point is that all of his books are awesome. I even liked some of the science stuff. ;-)
The first few chapters of his Lectures on Physics are suprisingly readable as popular science. Later on it gets more hard-science-y.
I'd say the next logical step is to read James Gleick's biography "Genius", which gives a slightly more well-rounded view.