Toward Clarity and Grace begins with a "history of bad writing" and an analysis of its causes. Five chapters build a model for structuring sentences, paragraphs, and whole documents so as to achieve clarity, cohesion, and coherence. (This is a complex model, complete with its own technical terminology — too complex, unfortunately, to summarise here.) Three chapters then describe elaborations on this basic structure, explaining how to vary concision and length as necessary and offering some hints on how to attain that indefinable, elegance. Only in the final chapter does Williams give "usage" advice, and there he stresses that most of the stylistic bugbears of grammatical pedants are actually perfectly acceptable.
Though his central framework is quite abstract, Williams provides plenty of examples to illustrate it, taken from a variety of sources. He analyses examples in detail and even rewrites some to illustrate the effects of different approaches. His presentation of both theory and examples works excellently; his own writing is a good advertisement for his ideas.
Some will not like its approach — anyone after a "learn to write well without actually doing any thinking" fix should look elsewhere —, but Toward Clarity and Grace is the most useful book on writing I have read. I have several friends (busy writing up theses) who would profit from it, but I am reluctant to part with my copy: it now occupies a place on my bookshelves next to The Elements of Style and The Chicago Manual of Style.