Masello's fifteenth rule is concerned with publishing waves.
If you're at all attuned to pop culture, you've seen this phenomenon happen dozens of times. A publisher comes out with a specific type of novel, and suddenly everyone seems to be publishing similar books. A wave has hit. The one that comes to mind most quickly is the wave of young adult books and fantasy books that were published in the wake of the Harry Potter series. But there's also the surfeit of conspiracy books that have appeared since the success of The Da Vinci Code.
Waves happen in all sorts of media. (Yes, I'm using my physics background to make a pun.) We see this a lot on television -- when Friends became a big hit in its first season, the next year the networks debuted a variety of buddy shows. Survivor launched a fleet of so-called reality shows. More recently, last year's success of Lost convinced the networks to go with other similar fare, which accounts somewhat for the debuts of Surface, Invasion, Threshold, and Night Stalker (although the latter two have been canceled).
So, at first glance, one might think that a good strategy to sell a novel is to write to the wave. But as Masello points out, there are factors conspiring against you if you try that. And the main factor is time. It can take up to a year to write a book, and over a year for the book to see print. By the time you've spotted a wave bearing down upon you, it's already too late to catch it.
And Masello further points out that the wave may not be the kind of book that is yours to write. If your strength is in mysteries, and you're really eager to write one, does it make sense to try to catch a romance wave? Masello feels that you're better off writing the book you want to write, not the one that you think will catch the wave.
And I agree with him. Don't sit down to write a book simply because you think that everyone is publishing that sort of book. If it happens to be the type of book you really want to write, that's okay, but if you're doing it simply because everyone seems to be into unicorn stories this year, or techno-thrillers, you're sure to miss the wave, and you may even end up with a more mediocre book than you could have written.