Written well after the original Foundation novels, Prelude to Foundation is one of two prequels. The book ties in nicely with Robots and Empire, as well as the other robot novels.
Its a good book too, with its 460ish pages only taking a few days to read (I think I knocked it over in about four days). The story is sufficiently engaging that I kept looking up and discovering that I had read another 100 pages. There are also a couple of twists in the book (I count three in fact), only two of which I had figured out before the characters.