The most important element of writing any book is, of course, its content. It also may seem the most intimidating to put those words to paper (or screen).
As an authority in your field, you probably have 80% of the content of your first book already existing on your hard drive, in the cloud, wherever you store your files. By finding and organizing these materials, you will streamline the writing process.
First though, you need to figure out what your book will be about. What is your message, your "Big Promise" you're making to the reader? What problem do you want to help solve for them?
Once you decide upon your subject matter, you should produce an outline -- think of this as your Table of Contents -- that will walk your reader through how you are going to deliver on the Big Promise of solving their core problem.
Next, do an inventory of all your content assets that relate to the theme of your book: white papers, sales copy, scripted speeches, blog posts, etc. By reusing, repurposing, and updating content you have previously created, you'll significantly reduce the amount of work you need to do to finish your book.
Having built your outline and preloaded your book with preexisting content, you'll also likely avoid the "stare"--i.e., when a writer opens up their word processor and just stares at the blank page, uncertain what to write about. Your streamlined writing process will chiefly revolve around connecting your recycled content, filling in the gaps, and smoothing out the edges.
After writing your first draft, you may feel you need to bulk up your book because it's too short. To do this, you can add client case studies showing your solution-solving in action. You may also consider including special chapters written by one or more of your colleagues, focusing on a specific facet of your business. (For example, if you're a realtor and your book is about buying and selling Real Estate, you might have a mortgage lender you know write a chapter on how to apply for home loans.)
Also look for opportunities to insert infographics about your subject. These are graphic visual representations of information, data, or knowledge intended to present them quickly and clearly. These can be a variety of tables, graphs, flowcharts, timelines, etc.
Keep in mind, your writing doesn't have to be perfect. (You'll pass your manuscript over to an editor and proofreader to polish it before publishing it.) Your primary objective to make sure your message shines through. As long as you fulfill the Big Promise you made to the reader, they will be satisfied and impressed.