Many, many years ago (in a galaxy far, far away...wait, that's from a movie, isn't it?), I came across a slim volume entitled The Elements of Style. The primary author, William Strunk, Jr., compiled the book from the lectures in his English classes at Cornell University. Legend has it that his "rules" were so specific that he had to repeat them three times each in order to fill up his course time.
After Strunk's death, his former student, E. B. White (the beloved author of Charlotte's Web and Stuart Little) was asked to revise and update Strunk's book. The agents asked him to add a section on style and usage. This little book is so well known that all you have to say is "Strunk and White," and writers know exactly what you are talking about.
Now, forty years after I first came across that book, I want to emphasize how important it still is. In it's 100 or so pages, it distills the rules of grammar, punctuation and usage to their essences. It's not a racy read; in fact, it is rather dry. But, if studied carefully, it will provide its reader with the basics of good writing. I mean, how can you improve on the admonishment to "Omit needless words"? That pretty much covers good writing!
This book is seldom revised since White's death, and, after all, why should it be? The rules that apply to writing on paper still apply to writing on the computer screen, whether we are discussing emails or essays. Good writing is good writing, however you do it.
If you buy only one "grammar book" in your lifetime, I'd recommend this one.