Books On Grammar: The Elements of Style and Advanced Grammar In Use

E.B. Whites and William Strunk Jr's The Elements of Style is an easy book to learn how to write well. http://www.amazon.com/Elements-Style-Fourth-William-Strunk/dp/020530902X

Steven King highly recommends this book, saying which is unlike many books on how to write, has very little or no bullshit!! Well, I've been reading it a bit, and I am willingly taking King's advice!

Another great book on writing is by Steven King himself, On Writing. I admire his simple and straightforward style.