Today's book market is more packed with choices than ever before in history. Instead of slogging through endless piles or pages of books, readers look for titles with familiar story ideas. If they liked one cozy mystery set in a yarn shop, they're more likely to pick up a cozy mystery set in a chocolate store. Put your cozy mystery in the inner city, and your fan base just goes down the drain. The best way to get your new book to stand out in a sea of others is to appeal to readers who are already looking for a certain kind of book. You do this by writing to market. 
Writing to market has gotten a bad reputation. It's seen as the enemy of the true art of writing, a mercenary act that uses the coat tails of popular books to crank out inferior copies for quick bucks. In truth, it's just the opposite. Authors who write to market find out what readers love to read, then give them more books in a similar vein. Not faint copies of bestsellers, but their own spin on the general topics their readers crave.
But the Art!
There are two kinds of authors, and you need to decide which one you are. Some authors want to write the book of their heart, the book they've been craving to write for a good part of their life. If it sells, great, but that's not their motivation.
The other type of author views writing and publishing as a business. They want to make a living from their writing. They still want to write good books that readers will love, but they want to use techniques that maximize their chances at making money from their work.
There is nothing wrong with either kind of writer. But if you want to treat your writing as a career, it's up to you to do all you can to run your business in a smart way. Writing to market will increase your chances by a wide margin.
What to Write?
If you're going to be writing full time, you might as well do it in a genre you like. Write down three categories you love to read and do the research on each one. You'll find a sweet spot between love and money somewhere in that list.
Check the listings on Amazon for each category. You could use Nook, or Kobo, or iTunes, but Amazon sells more books than any of them, so do your research there. Find a favorite category, and drill down to the sub-categories underneath. Look under Mystery and you'll find Crime Fiction, Thrillers, and more. The same goes for all of the main categories. Drill down even further to find smaller specialized categories: Crime Fiction to Organized Crime or Serial Killers. One of these smaller groups might be the one you're looking for.
Check the rankings of the number one book in the group. You can find the rank about halfway down the page. Ideally, the number one book should rank under #1,000. This means a lot of readers are buying this every day. Books of this type are hot, especially if #2 and #3 are close in ranking. Next, look at the ranking of the #20 book in the category. If the numbers fall off steeply by this number, it's a good sign that readers aren't finding enough books they like in this genre. You've found a sweet spot.
It's Not That Easy, Is It?
Finding your genre is just the beginning. You've also got to consider tropes and reader expectations. Mystery readers expect that the mystery be solved. Romance readers insist on a happily ever after ending. Cozy mystery readers expect a drawn cover with a cute illustration, while paranormal romance fans want a bare man's chest along with a wolf or bear. These are just expected parts of their favorite genre. Read at least five books in your genre and take notes, writing down all the tropes they have in common. Look at the covers, the chapter length, the style of the blurb, and even the font color on the cover. Make sure your book fits in with the other best sellers like a set, not like a carbon copy.
Above all, you have to write a good book. Craft is king, but writing to market is one of the best ways of making sure your work gets to the most readers possible. After all, the best book in the world is no good if no one reads it.
Keep the Faith and May the Force be with You!
