Lessons learned: writing really long fiction

On the one hand, publishers marketing departments love a series: if I hand in a new Laundry Files or Merchant Princes book, my editor knows exactly how to tell the marketing folks to sell it. The Empire Games trilogy is what the third book in that series became, over a 15 year period—it bears little or no resemblance to the original pitch, and the fourth book in the original pitch will probably never get written because the actually-existing series has diverged too far from the original plan. So the original series plan may no longer even be appealing or interesting to you if you try to stick to it religiously—you need to be able to redesign your project on the fly, taking into account the already-published books that are already set in stone, if only by layering new interpretations of old story elements on them.

Source: www.antipope.org