What are you selling? Who's going to buy it?
Convincing people to buy your book is your greatest challenge. It helps if you think of your book as one part of a larger selling strategy. This larger strategy has to do with selling yourself. After people begin to see you as a trusted source of information or entertainment, they will also buy your book, attend your seminar, purchase your consulting, visit your site, read your blog, and refer friends. OK, maybe they'll do just two of these things, but it all helps to move your strategy forward.
Here are some ideas on how to complement your career with publishing.
Common topics are pretty well buttoned up by hundreds of popular books. A new book on Photoshop must compete with the 800 other books about Photoshop currently available. Consider aiming at a niche market instead. For example, an Amazon search suggests that only a handful of books are dedicated to scripting Photoshop. A shopper who wants to read about scripting Photoshop certainly would investigate these conspicuous titles.
As a professional, you probably have a specialty. It makes sense to highlight your specialty using a specialized, niche publication. It will be easier to write, and it will enjoy a distinctive profile in the market.
Write about an emerging technology or trend. Or, write about a recent event. Such publications might look more like reports than books, but shoppers will appreciate your timeliness. A related service has sprung up at http://www.LJBook.com. It creates a printable PDF from your LiveJournal blog.
If you are a consultant, publish a book that discusses the most common problems you encounter in your field. Think of it as a consumer-grade product that complements your high-caliber practice. Your status as a consultant will fuel book sales and your status as an author will fuel your practice. For example, the folks at http://www.irwebreport.com use a combination of gratis online articles, for-fee reports, and consulting services to advance their industry profile.
If seminars are your specialty, publish a book to sell to your students, fans, and peers. Selling a tangible book will complement your intangible seminar product.
In short, publish a book as part of your larger strategy to sell yourself. Sell yourself by participating in and contributing to your community.