[Tomte]

I second that.

It has two things that stood out for me:

* It clearly discusses "grammar rules" and "personal style", which helped me locate myself in all this.

* It doesn't just say "avoid the passive voice", but explains when the passive is needed (it has nothing to do with "when we don't want to say who acts", but a lot with "the sentence is easier to understand when X appears at the beginning of the sentence, the easiest way to achieve that is to use passive voice").

Oh, and a third: it has wonderful exercises.

There are many editions of the book (actually there are even three titles, all with "Clarity" and "Style" in them) – it doesn't really matter which one you choose.