The first level would consist of the avoidance of mechanical errors. Writing as the avoidance of mistakes in spelling, punctuation, and grammar. This low understanding of writing puts faith in simplistic rules, like avoiding the passive voice or not using wordy phrases like "the fact that." The first level is often devoted the shibboleths and zombie rules like split infinitives and sentence-adverbial "hopefully." Advice oriented toward this level of thinking is often misleading or mistaken. It is reactive and peevish. Since many bad writers overuse the passive, let's ban the passive voice! Since good writing tends to be concise, let's eliminate any unnecessary words! The Elements of Style is a good representative of this kind of thinking.
The second level would entail a more accurate understanding of language, distinguishing carefully between usage, register, and grammar. It would recognize that every good writer uses the passive when it is the best option. This level is oriented toward developing a serviceable style, clear and unobtrusive, rather than simply the avoidance of error. Joseph Williams or Claire Cooke are good writers to follow on this level.
A third level looks behind the surface of style and sees writing as the expression of views about the relation of writer to audience, language to reality. Now style is at the service of other values. Turner and Thomas address this in their book Clear and Simple as the Truth. This is a good book even for those who don't like the classic style that these two authors advocate, because it clarifies some basic issues.