To write a great novel, read. You must read widely, and specifically. Read the genre you wish to write, and the genres you don’t. Learn what works, and what doesn’t, and you, too, can write something that will sell. Or can you?
Might you not, unintentionally of course (for who would want to plagiarise?), simply end up writing something which is already selling, and for which there is a finite appetite in the buyers’ world?
Now, obviously, I have no authority to speak on this. I’m not a published author (yet, one hopes), but I am a reader. And while I read what interests me, what that is varies on a daily basis. It can easily vary within the hour, unless I haven’t finished what I started or if it is so thoroughly tedious I can’t cope anymore. Murder mysteries with lots of needlecraft before breakfast, historical romance at luncheon, and fantasy by supper-time. While I can read lots of a single author, if I read too many similar such books, I do rather go off the style, or stick with the original. So writing to the market is not necessarily the route to success, although I have no doubt that it is a good route for some.
My suggestion, as a reader and aspiring novelist, is this: Read widely, learn how to use language, and find your own ‘voice’. Never mind what sells at the moment. That’s today. Tomorrow it’ll be something else. With any luck, tomorrow, it’ll be your work which sells. Don’t look back, look forwards. Write the story in your head which demands to be written. Your characters.
My novel, which I’ve just completed (but not finished; it needs a lot of editing), is a fantasy novel. Which is odd, because that’s not a genre I necessarily read a lot of. I don’t ignore it intentionally – until about book 5, I loved the Harry Potter series – but there are few fantasy books which really grab my attention. Perhaps I have simply yet to find the good authors, who don’t just have tired orphans being the lost heirs in kingdoms taken over by over-the-top evil sorcerers. (If anyone can suggest some which aren’t like that, I’d greatly appreciate it! Preferably not door-stopper books, though, please.)
Actually, the not-reading-fantasy makes it a little difficult for me to know how to describe my story. It’s not epic – no wars or kingdom-fighting; the magic is limited; there are but few magical races; and the most of it is centred on the doings of the new, youngest-of-six-sons, baby-of-the-family King. The Fates, no doubt, had their reasons for choosing him, but the watching Gods, who can do nothing and have no control, can’t see it. And they’ve been watching for a long time. They, perhaps, have a decent grasp of consequences, which the Fates, in their desire for amusement, have ignored.
But it is the story which demanded to be written, which claimed my attention and refused to be ignored, ignoring my normal reading habits. Whatever you do, don’t let someone tell you that you can’t write something because you don’t read it. Maybe if your story was out there, you would read that genre. Just make sure you know your story and your world better than anyone else. If you know it, you can write it, regardless of your reading preferences. Good luck!