Kennan’s book was published as an alternative to the Piston text when it was becoming outdated. Then Norton updated Piston, and since then Prentice-Hall have updated Kennan, and are now competing against Adler with a CD set and so on. It’s all useful, but not as usable as Piston in Adler in terms of organization of information or writing style. One good point is its obsession with phrasing and bowing, going into great detail about both. But it’s not essential in my view, though many of my colleagues love it.