Do note however, that when I say what has been selling, I'm referring to not only books being sold in stores, but also e-books, and books being contracted by authors and agents to the publishing houses. I differentiate these three because the timing on when they are written and published varies significantly, and if you are simply following the trends of what is selling on the best seller lists now, you'll be completely off base.
We often tell authors to write the story that they want to write. And write about what they know. These two are, more often than not, prerequisites to a good story. If you write about something you want to write about, it will likely be a story that you enjoy and are more likely to dedicate yourself, think the details through, and enjoy the process. It isn't just more fun, and more productive, but the end product is usually better when the author is loving the story as they are writing it.
Writing about what you know is also easier and results in a better product because you already know what you're talking about and will need to do less research, and will usually tend to be more accurate and authentic.
I often find that when someone writes about a topic or place that they are not familiar with, small errors happen, and the material can sound forced, bringing the reader to question the authenticity of the facts and perhaps even disconnect them from the story.
Additionally, when the author spends too much time doing research, there will be a stronger tendency to want to include as much of that research into the story to show the reader that they know what they are talking about. They end up dumping chunks of information into the story that lacks flow and disturbs the tempo or pace of the story.
If that's you and your story, great, but you may still want to try to figure out if the story you are writing or want to write is going to be marketable and synchronized to the trend wave when you're ready to put it out there. If this is the case, then you should be looking at what's has already been published, and what the demands have been and are, to determine what will be in demand... or so out of the proverbial box, that everyone will want it.
I mentioned that we should differentiate between books based on timing. Let me expand a little bit more on this because too often I've seen and heard people try to work out their timelines for writing, pitching and releasing based on inaccurate presumptions.
Books on shelves, books on lists, books coming out, books being acquired by publishers were all written at different times. If you are trying to use this information you need to look at all books within a period of time in order to see the big picture from what has been selling to what will be out in the next few years, in order to try to guess what will be hot next.
To figure out what has been selling, you should be looking at the best seller lists, reviews of purchases on Amazon and the such, book shelves and e-categories. The best seller lists going back as far as you're willing to extend your research will show you the most popular books with the best sales on a weekly basis. To do the best research, you should actually look at when that book was published (and the copyright notice on the first few pages because it may be a rerelease), and if you really want to figure out the timing, take a look at when that book was acquired by the publishing house to see how long it took them to release it. That's when you will start to get a full picture.
Like I said above, working with an agent may be necessary and always valuable, but it will add time to the equation. Most top publishers don't even look at un-agented work. Here are two ways to get around this issue. One is to find large publishing houses that do just e-books or e-first. Most of them now have imprints that do just that. These imprints may look at unagented work, and the releases are faster because e-books are easier to produce. The second is to pitch to a publisher directly at a writers conference and if they like it you can then more easily find an agent to help you with the contract.
So when you want to try to figure out a trend, it's smart to know what's out there and what's going to be out there. If you take into consideration what I just said about big publishing houses, you'll realize that you can actually figure out what books will be publishing in the next six months to a year or so, by looking at the listing of deals made by publishers. This is available on Publisher's Market Place.
By looking at all of these sources, now you can have a better grasp of what's happening in the industry. Granted, this does not entirely tell you what independent authors will be publishing, but if you follow some independent authors on social media, and see what they are publishing, they are fairly transparent. You eventually start to get an idea of what to expect from them as well.
In way of example, there are three or four bicker series that have just been started by successful enough independent authors. This should be something to keep in mind, because this should tell you that within the next year, they will all be putting out biker books months from each other. You need to be able to see that having these many books on bikers at once, will have an affect on the market. If you have a biker book, your decision should be to either publish it at the same time independently to ride the wave, or just hold off until they have finished their series and see if you can pick up their fans when they are done. When doing the latter, of course, you risk being too late. People may be sick of biker books by then, and all of the books on this topic may have already been sold. And so if you don't have something with a unique hook, you may hold off on that story all together and cut your losses. I don't make this suggestion lightly, but business calls are often hard to make. And good business people need to know when to make them.
The same thing would apply to starting to pitch these types of books to agents or publishers, because by the time those books would release, the wave will likely have crested as well... if they even look at the books at all. Because agents and editors have a fairly good grasp about all of this and what they should be looking at. So if you get consistent declines on your work telling you that they already have similar projects in the work or published, then that should be your red flag.
Therefore, even though there really are no rules, and you really never know what will work in the market, because there are so many examples out there that have had unexplainable success. And there's no scientific way to know about new ideas, it is imperative to read and research the market to know what's been published, what has been doing well, what is coming out, and what will be out in the near future, because having this knowledge about the market will allow you to be able to determine with some logical certainty, whether and when it is the best time to pitch or publish your book.
Making an educated decision is always better than shooting in the dark, and then wondering why your didn't reach your target.
Publishing is a business industry, and that makes you a business person. You need to treat your work as your coveted product, and to sell it well you need to do your research and know as much as possible about the market and its players. Only then, you'll be able to make the best decisions for your brand and your product.