The time will come when your sales growth will start to slow. Eventually, it will most likely level off and start to decline. Only a handful of books become true "classics", and even those don't sell at anywhere near their peak rate.

It's important to keep a very close eye on your sales rate, because as soon as the growth starts to slow, it's time to think about diversification. This can be done in several ways:

• New editions — If the core concepts are timeless, and just the references are out-of-date, a new edition may be all that's required. Rhonda Abrams' The Successful Business Plan: Secrets & Strategies, for example, was originally published in 1991, but has remained timely with revised and updated editions in 1993, 2000, and 2003.
  
• Re-purpose existing material — If you haven't done so already, now is the time to develop a workbook, an audio book, an audio course, etc. Of course, I'd recommend doing it sooner rather than later, but this will substantially extend the life of your existing material.
  
• Diversify the core concept to new markets — Perhaps the most familiar example of this is the Chicken Soup for the Soul Series, which now boasts dozens of titles for every conceivable niche market. While this example might be a bit extreme, certainly creating one or two extensions of your book for particular niches is not, and is a great way to extend the life of your material. You don't even have to write it yourself. For example, The Chasm Companion was written by Paul Wiefels, a colleague of Geoffrey Moore, author of the high-tech marketing classic Crossing the Chasm.

Conclusion

Don't let your book become a tax deduction. Treat it as a business from cradle to grave. Know your market, build a network to support the creation and promotion of your book, create and execute a realistic marketing plan, and know when and how to move on. Do this, and your book can be a lucrative venture of its own, as well as a great promotional tool for your other ventures.