Use your book to establish your credibility...
Many professionals consider writing books as a calling card, and I do believe that is quite effective. However, that was not my motivation. I’m writing books, because I’m serious about a long-term career as an author. That said, being an author has been beneficial for my other professional pursuits because of the attention it brings.
Author of Self Marketing Power: Branding Yourself as a Business of One
Use them as introductions...
I've been amazed at how many books we have given away as a way of promoting our nonprofit organization. Not only has the book given added credibility to what we are doing, it gives people a chance to get an in-depth picture of how our organization fits into the larger social structure within the community.
Co-author of Coaching for a Bigger Win: A Playbook for Coaches
Handing a potential customer or client your own book solidifies your place in that person's mind as a recognized, knowledgeable expert... and a generous one at that.