One of the biggest mistakes writers—both published and unpublished—make is trying to game the system by picking a project based on what they think will sell, instead of writing something they’re excited about. From my own personal experience, this is usually a one-way ticket to a terrible novel no one wants to read, much less pay you money for.
Your mileage may vary, of course, but the writing you do for passion will always be better than the writing you do via market research and trend-watching. Smarter writers than me might sell those novels, of course, while I wallow in my for-the-love paradise, but personally I think when you write a better book, you sell more copies. My experience has also been that the books you sell yo publishers are rarely the books you think you’ll sell to publishers. Again, your mileage may vary on that, but so far every time I write something that’s a surefire hit, no one wants it, and when I casually toss a trunk novel onto the table with little enthusiasm, it comes back to me with a contract. The world is mysterious.
The Patterson Effect
Some might point to writers like James Patterson, who seems to have the “write for sales” formula in lock—and he certainly does, along with a few other writers who can churn out 4-5 novels a year (often with co-writers, who are likely doing most of the actual writing) and hit the bestseller lists with most of them. This is a thing, of course, but consider that Patterson worked a long time in a more traditional way, building up not only writing experience and craftsmanship but publishing-specific business experience as well as relationships throughout the industry. He didn’t just decide one day to become James Patterson, Inc. It was a process.
Of course, for many unpublished writers the urge to try and crack the code so you can write a book genetically engineered to sell is a powerful one. Just keep in mind how many novels are bought and published each year, compared to the ones that actually sell enough to warrant second novels. The gap there is huge, and the lesson is easy: No one knows what’s going to sell. Publishers make educated guesses, and are often wrong. So the chances that you’ll guess right are even slimmer. Just write a book you’d want to read. It’s the strongest play.