Writing a book takes longer than commercial writing, but the problems faced in getting started are virtually the same. Most writers start with some form of procrastination; so don't think you're alone. Just start writing and you'll be surprised what comes of it.
In writing a book, you need to do a little more planning and organizing. What is the subject of your book? What are you trying to say? Who are you communicating with? Who is your target audience? (Well - I guess you ask yourself all those questions, too, when you're writing an article.) The point is, have some sort of plan in place. Whether it be for an article - or a chapter.
Assuming you have an idea, begin writing free-form getting all your thought down. Don't worry about spelling or editing at this time. The important thing is to get the ideas down on paper (or computer 
).
At this point, you're writing a draft. In the case of a longer book, it may only be a chapter or two. Don't get stuck on details that cloud the overall idea. Just write.
Develop a pattern. Find that time of day where you are most productive and set aside a block of time just for your writing. No phone calls, no snacks. Just write. The writing process will vary from writer to writer. Find your comfort zone and make it a habit to work during that time frame.
The more you write, the more you'll improve. Don't let writer's block (sometimes called procrastination) stop you. Don't over complicate the process. Allow your thoughts to flow freely (unencumbered with worries of spelling or grammar) to start. Worry about editing later.
For new writers especially, it's important to stick with it and develop a routine. You'll be writing faster and better in no time at all.
Check out the Novel Writing Made Easy System.
