This piece of advice isn't new. You hear it over and over again. The best way to market a book is to write another one -- and then a third and a fourth. To prove the point take a look at supermarket and drugstore paperback books. The same author names appear over and over again. Best-selling authors may have two or more books in the top ten lists at the same time. And the explanation is simple. Authors become popular, not because they sell more books, but because they write more books than their contemporaries.
Think like a reader for a moment. How many times have you finished a book and wished you could lay hands on the next one immediately? Have you ever wished you could tell a writer to hurry up and publish the next volume? What's the first question authors are asked when they appear at a signing or book talk? Chances are, the question is "When is the next book coming out?" And until a new book appears, the fans of one story will go hunting for older volumes they may have missed. That's when your books take on a life of their own.
When I look back at my own reading habits, a clear pattern emerges. I nearly always chose the thickest book I could find, because I knew that once I started reading, I wouldn't want to stop. Early favorite authors were (and this is really going to date me!) James Michener and Carl Sandburg in his novelist period. Both produced great sweeping sagas of history, following the experiences of one family through multiple generations. A bit later I devoured the Brother Cadfael series (Edith Pargeter), the Plantagenet tales of Thomas Costain and Sharon Kay Penman, and Diane Gabaldon's "Outlander" series. In each case, the newest book had its own conclusion but carried the hint of further stories to come. And I wanted them all--right then! How frustrating it was to finish reading 800 to 1200 pages and know that there was still more to come. The only thing worse was falling in love with an author's characters, only to find that the author himself (like J. D. Salinger) had quit writing entirely.
Looking back now, from Michener's worlds to the Harry Potter series, I can understand those long waits between books from the author's point of view. Readers only want to know what happens next. Authors, however, must deal not only with the demand for more but with a feeling of responsibility to the characters created. I first felt the pressure with my current release, "Damned Yankee." My editor sent me her final revisions, with a comment that I needed to start the next volume because she wanted to know what happened next. Shortly after the book's appearance, a reviewer mourned that she was going to miss the characters she now knew so well. The audience was waiting; it was time to start writing again -- for both my readers and for me.
So, a final lesson learned: Keep writing. Your audience is waiting, and your series will create a living and growing community of readers.