If you don't think you're capable of writing a book, you're in good company. A lot of authors feel this way; I know I do. Even after publishing three well-received business books, with a fourth on the way, I still don't feel capable of "writing a book," at least not if I let myself think about it as a single, fearsome entity.
In fact, I'd suggest there's little that's more crippling to a writer than to set out in the morning with the daunting goal of "writing a book," and that there are very few authors who are capable of doing such a thing when you put it in those terms. Instead, we keep our eyes on the work and the self-intimidation level low by writing sentences, paragraphs, chapters; then, one day, we have something that resembles a complete and bona fide book. It's rough, practical magic, but it works.
(I'm not trying to understate the importance of being able to properly put together a book. All of those paragraphs, pages, and chapters that I'm suggesting you work on piece by piece? They can't just be put together haphazardly. The actual book does need to be an actual book, not a collection of chapters. For more on this, I refer you to "Structure," a striking essay on how to bring structure to your nonfiction by John McPhee, one of the greatest nonfiction writers of our time, and to this fabulous full-length book on the subject by nonfiction great Tracy Kidder and his longtime editor, Richard Todd.)
There are authors, of course-many of them, and almost all of them more storied than me-who don't follow the piecemeal pattern that I've just laid out. These authors, it could be argued, come closer to organically writing a complete book than do those who write the way I do. I'm talking about J.K. Rowling, perfectly mapping out, in the course of a single train ride, the plot strands that would take her characters through the course of multiple novels. Or the novelist Ann Patchett, who conceptualizes her works entirely before she types a single stroke on the keyboard.
Yet even such writers, though they come up with an impressively complete concept early on, aren't creating their books so entirely all-at-once as it sounds. Although they may get down the grand scheme of things all at once, there are still the sentences and paragraphs to write, the dialog (if any) to make sound convincing, the adjectives and verbs and adverbs and nouns to pick and re-pick and cross out and re-pick again. Finishing the concept doesn't mean finishing a book; far from it. As Patchett puts it, you still have to "make all the trees and all the leaves and then sew the leaves onto the trees."
In fact, if you look at a dramatic story like Rowling's more closely, it looks grittier and more piecemeal than it might at first blush.
Here's the part of Rowling's story that's the stuff of legend:
I simply sat and thought, for four (delayed train) hours, while all the details bubbled up in my brain, and this scrawny, black-haired, bespectacled boy who didn't know he was a wizard became more and more real to me.
And here's the less sexy part of Potter's origins, the part people overlook. Although Rowling "began to write 'Philosopher's [Sorcerer's] Stone' that very evening," she concedes that "those first few pages bear no resemblance to anything in the finished book."
So what I'd encourage you to do today is to give yourself a break on this whole book thing and spend your energy instead getting to work. Even if you don't feel capable of writing a book, it is possible to write a sentence. A paragraph. To pick out an adjective, to tweak that outline. And keep moving, moving, moving 'til you have something that resembles, at last, a book. Written, somewhat to your own immense surprise, by you.
There's a broader point here, if you don't mind me moving past writing as writing and on to writing as a metaphor. The people who are successful, not only at writing books but at starting companies, building organizations, mentoring hard-to-reach students, and other daunting tasks are the ones who don't psych themselves out by thinking they have to get it all done at once. Instead they get these projects started, and trust that they'll find a way to finish them via the momentum that they generate along the way.