Publishing is full of conventional wisdom but woefully lacking in data to support many of its theories. In the professional services field (management consulting, training), the general rule is that if you publish a book, you will get more engagements—speaking and otherwise—and command higher fees. Mike Schultz, publisher of RainToday.com, a company that works with professionals to help them market their services, said the lack of proof that a book could boost the earning power of a professional prompted his firm to do its own study. While "The Business Impact of Writing a Book" is geared to business book authors, its lesson are applicable across a range of book segments.

The key finding of the report is the payoff that comes to authors who invest their own funds in promoting a book. The study found that 51% of the 200 authors who took part in the survey used their own money to back their book, with the median investment $4,500. The form of investment typically involved hiring a marketing or public relations firm, and the impact these companies had on a book's sales is significant.

Comparing the average response, the study found that authors who had outside help sold 10,000 copies of their first book and earned royalties of $55,000, compared to 4,500 copies sold and royalties of $25,000 for authors who depended solely on the efforts of their publishers. Not surprisingly, 85% of authors who used an outside firm said they would do so again, although a few authors had some words of caution. "Beware of PR agencies. Check them out thoroughly before you sign up and have a cancellation clause," advised Lydia Ramsey, author of Career Development Strategies.

The most effective way to promote a book, the survey found, was through the Internet, followed by coverage in trade magazines that report on an author's particular field. Book signings were far down the list, though Schultz acknowledged that a consultant is more likely to draw consumers at a professional event than a bookstore. Still, some of the comments from business authors had a familiar ring. "The one thing that I could say was a clear waste of time was going to a bookstore and giving a talk," reported Ford Harding, author of Rain Making.

And while many authors lamented the lack of marketing support for their books, the survey found that the bigger the publisher, the higher the sales. An average author at a major publisher sold almost 11,000 more copies of his book than an author who was published by a small press. Authors at mid-sized houses sold only slightly less than those at the larger houses, while self-publishers sold far fewer copies than an author at a small publisher.

And how did consultants who published books fare? According to the study, 96% of authors said publishing had a positive effect on their business. The biggest gain in generating revenue, however, came not from royalties but from indirect income, such as additional client work; gains in indirect income outpaced royalty payments by a three-to-one ratio.