Make sure your book is really about something.
Books--like blogs--can be boring, unoriginal, lack a theme or direction, and sometimes they are not even about a problem. Don't be that writer.
Pick a topic based on something you really have a passion for. People want to travel excitedly with you. Otherwise, the laundry awaits. You don't even have to have a thousand things to say about it, if your idea is really clear and compelling.
Take, for example, John Gray. I've read two of his books, and they are so plainspoken and internally repetitive that I was able to read them in twenty-four hours each. He took a great idea that meant something to him, previously written about in a scholarly style by Deborah Tannen, and made it accessible to the rest of the human race. Now, thanks to him (whether they like it or not), most of the world is familiar with the idea that men and women have some fundamentally different relationship needs that they communicate differently about, too.
Which leads me to the second aspect of topic choice: being original. Seriously, Pema Chodron did not steal your idea. Be familiar with the rest of what's been said on your topic of interest. No excuses. It's easy to research it using the internet. Then ask yourself how your idea is different, or whether you can present an existing one in a way that makes it more accessible, or--if you're ambitious enough to enter the world of smartphone apps--more fun!
Now, what is your theme and where do you want to go with it? You have a collection of wise writings (yours, of course) that are unrelated. Stop. What is the thematic thread that runs through these pearls, that you really care about? Make a collection around this theme, and take longer to write more related entries, if necessary. Then give the book some organizing principle that will make it easy to navigate, such as the daily calendar, or a specific title to each entry that you can list in the table of contents. If all you have is a collection of somewhat random inspirational writings, with no way to find a particular one, then that and a beautiful cover will make a nice coffee table book for a few bed-and-breakfasts. But it is not a cohesive self-help book.
Lastly, a compelling self-help book tries to solve a problem. In fact, more and more, the public and its concerns are driving topics experts write on. Psychotherapist Mira Kirshenbaum (in my opinion, the best published self-help writer out there) crowd-sourced her latest--upcoming--book's topic through her website. Shoot, it may not be too late to give her your opinion on her cover design, if you go over there right now! (I'm not kidding.)
At the very least, ask your friends what they'd like you to write on.
They'll be eager to see it.
