The book is The Writer's Workshop: Imitating Your Way to Better Writing, by Gregory L. Roper. He says that writing can very effectively be learned in the same way that art or music is: my studying and imitating the masters' works. In the introduction he sketches out a scene where an aspiring artist and musician study the greats and expand into their own style from there and then goes on to say:
I give you these two vignettes because in them is bound up the central method of this book: imitation of the great artists as a way to learn an art and develop into an accomplished artist oneself. It is, in fact, the way almost all crafts, all arts, have been learned throughout history, from blacksmithing to shoemaking to jazz to, yes, writing. Shakespeare, Chaucer, Milton, and countless others learned the art of writing in a school system that, in teaching Latin, forced these students to translate and imitate the great classical writers of antiquity, over and over again. ...
Now, few writing books on the market today teach you to write by making this method an important step in learning. Some of them suggest that if you just express your inner self enough, through free-writing and such, you'll figure out how to write well just by constantly saying what's inside you. Others explore writing as politics (usually from the left), and encourage you to see writing as a negotiation between your situation and the power struggles around you. Still others tell you about the basic forms - of assembling an essay (intros, bodies, conclusions), and of argument (definitional, casual, evaluative, etc.) - and have you try your hand at assembling the writing through working on mastering those forms. All of them have a grasp on a piece of the truth, but all of them, as G.K. Chesterton says of heretics, think their piece of the truth is the whole truth. (Pages xiv-xv)
This classical style of instruction makes a ton of sense to me. In fact, it's the philosophy that I'm already trying to use in our homeschool. (Incidentally, the commentary on this particular reason to study Latin is just one of a fistful of persuasive reasons that has me seriously considering adding it to our line-up at some point.) When I have Hero practice his handwriting I will typically choose a sentence from our read-aloud and have him write it out. Not only does he get to practice the nuts and bolts of letter formation and usage rules, but he is also (I hope) internalizing what great writers do. The way they build their sentences, the pace and rhythm of it, the vocabulary and style. Some of that will happen just in the process of reading the classics, but with the copywork, and later with exercises like the ones in this book, I hope to help my kids grasp at least the basics of what made the great writers so great. And I want that for myself, so I'm going to do it as well.