There are two kinds of writing supplements: some explain language mechanics, and some teach the elements of style. Getting the mechanics (punctuation, grammar, etc.) straightened out isn't much fun, but it's necessary. Exploring style is lots of fun, but can be just as frustrating.
Even the best writers often have trouble teaching others the craft. Teaching writing, it seems, isn't intuitive. Like many skills, when one gains proficiency putting words on paper (thoughtfully and creatively), the steps to get there are often forgotten. The books in this section are intended to help instructors teach, and students learn, the basic elements of good writing.
Any Child Can Write is a classic manual for parents wanting their kids to develop good writing skills from an early age. Harvey Wiener shows how even a shopping list can provide excellent practice for burgeoning writers, and how exercises don't need to be overly elaborate or stressful to be valuable. This is a good book to read before your kids even get to the writing stage, though it's useful at later levels as well.
Using the same methodology that made her Easy Grammar series popular, Wanda Phillips' Easy Writing teaches elementary, middle school, and high school students the basics of sentence structure through repetitive exercises that implant and cement information in kids' minds. Jump In goes further, by offering step-by-step instructions for composing creative essays (this is a popular product and highly recommended).
Many of these resources are workbooks, showing kids how to compose an outline, find a topic, take notes, conduct research, etc. Titles like The Write Stuff Adventure are less about composition and more about finding something to say and knowing how to express it. Such books are especially helpful for students who complain about writer's block.
Some now-standard reference works stand out: The Elements of Style by E.B. White and William Strunk, Style by Joseph Williams, and On Writing Well by William Zinsser all offer excellent advice for those trying to find their own effective voice. The newer Eats, Shoots & Leaves is a more mechanics-oriented book on a similar theme (how punctuation affects what you're trying to say).
None of these materials should be looked at as simply for those who want to be novelists or famous writers. They're for everyone, to improve your writing and help you say what you want in the best way possible. Writing isn't a school subject just so you can get A's on your papers; it's an essential communication skill that will determine your success at work and at home for the rest of your life. Get good at it!