Would you try to make a movie without watching some movies? Lots of them? Thousands? We have to do everything a movie does. We have to capture the same audience. The movie maker has millions of dollars and a promotional budget, actors, special effects, etc.
We have some carefully arranged lines and curves on paper. Best make the most of them.
Best read some books. Lots of them. Thousands. Worried you won't have enough time to write? Don’t. Reading counts as writing time.
Get to know the recent award-winning and notable books, but don’t stop there. Make sure that a good part of your reading diet is composed of both classics and recently published works.
If you're writing for a specific age group or about a specific theme, become well read within that age and subject-theme category. You'll get a first-hand feel that no how-to book can provide, a hard look at the "competition," and a sense of what's passé. You'll be able to identify publisher lists that seem to fit your style and to rule out any that might already be featuring a too-similar book.
Don’t become easily discouraged if you notice a variety of books that tackle the same theme, especially if they don’t cover all age ranges. There is always room for excellent stories, so long as your twist, your breathing characters, and your voice keeps them fresh.