Beyond Strunk & White, there are few classic references for the basic copyediting that so many authors are ill-equipped to do. Line by Line is one of the classics, and it's something one should read through every four or five years. (I say that having only poked around in it, previously. This is my first time going cover to cover.) Just as I do, for instance, with Strunk & White.
It addresses the very basics of effective rhetoric at the word and sentence level (alas, it does not rise to the paragraph): how to tighten sentences; how to conjoin parts of a sentence; how to keep parallel parts of a sentence parallel; how to police errors in number and reference; and how to master problems of punctuation. There are two vital appendices, which go beyond dictionaries, but which take up less space than full grammars or usage manuals: one on the parts of a sentence, and how they work; one on problems of usage.
What I admire about this book is that it is based in the practical work of copyediting, and the actual problems that come up. It talks about the debates among experts, and historical trends, and about context. Cook is not too quick to insist on a rule, but isn't afraid to point out that it's better to be safe than shame-faced. Neither a rules-Nazi nor a linguistic anarchist, she.
The book can be dry, but it's darned efficient. 205 pages covers a universe of sins, so it's worth sticking with it. I wouldn't advise reading more than one chapter at a time, though. The brain needs rest.
The reason I'm only giving this four stars has nothing to do with initial quality, but with the passage of time. This book is 32 years old, at this writing, and usage, especially, is a moving target. Note that she relies heavily on the Oxford English Dictionary and Fowler's Modern English Usage, which are both fine works, but which are both about British English. Eighteen years after this book, we finally got a reliable American usage manual, Garner's, and despite its historical gems, Americans should put their Fowler'ses aside.
Which means, I think, that it's time for a Line by Line update.