This volume is designed for those who have college-level reading abilities and can already write acceptable 500-word essays, but who are striving to write better.
Aimed at students but most of the material applies to any kind of nonfiction writing (technical, reviews, journalism, etc.). Less useful for creative writers, though I'd still recommend it for some of the heuristics and the overall emphasis on evaluating rhetorical contexts/discourse communities. Glad I came across this in a little free library.