When I hear from first-time authors wanting to write their nonfiction books, many of them say they have at least two or three books in them, and they need help pin-pointing their best material.
As a ghostwriter, I almost always recommend they write the "airplane read." When it comes to personal growth, business, memoir, and activism books, readers don't want another tedious 250-300 page book. They want 150 pages--something that takes them a few hours to complete.
The "airplane read" lets you release only your highest quality material and allows you to learn which chapters and themes resonate most with your readers. With that information, you can go on to (confidently) write book two, three, and four.
From a reader's perspective, a shorter book gives them something that's accessible, affordable, shareable, and digestible. When they whip through your book, believe me---they'll go on line and brag about it. They're also more likely to offer an Amazon review and testimonial, which you will need for marketing.
Writing a book takes the average author YEARS to complete, but I'm telling you--break it down and give your readers the gift of brevity. Give them the gift of getting to know the very best of what you have to offer.