One of the first pieces of advice a writer is given is to "know their audience". For whom are you writing? What age group? What genre?
Many times, if you look in the mirror, the answer will literally stare you back in the face.
Author Toni Morrison said, "If there's a book that you want to read, but it hasn't been written yet, then you must write it." Another way to look at this is, if you want to write a book, but you're not sure what you want to write, you must look at what you like to read.
Even children's book writers must look at themselves when they were children to see what kind of book they would have enjoyed reading at that age. When I was a child, I loved reading books that were about children just like me (the Little House books, stories by Beverly Cleary or Judy Blume) or in settings with which I could identify, even if some of the animal characters were gentle fantasy (E. B. White's novels, especially "Trumpet of the Swan".) I was never into science fiction or fantasy and I doubt I could have really enjoyed the dystopian fiction that is so popular today (let's not even start with the whole vampire genre), so it stands to reason that, if I wrote young adult or children's fiction, I would probably write books like the ones I enjoyed reading.
I write mysteries that appeal to people who live fairly uneventful lives--some would even call their lives "boring". Work, school, laundry, bills to pay, daily interactions with family, friends, and co-workers make up the fabric of their lives, just like those of the characters who populate my stories. Some of my readers appreciate reading about characters whose lives have more drama than their own--very few people would want to discover a dead body in their own home or business! Others, whose lives are fraught with a lot of tension and drama, enjoy a story that has a sense of peace and order when all is said and done, with love and loyalty being treasured character traits. I see myself in both camps, through different periods in my life, and I write what has always appealed to me.
Perhaps one of my earliest "audience members" was my late sister-in-law, Dawn. Twenty years ago, when we were living all together (her family and mine) in Mt. Kisco, NY, we both had developed a love for Mary Higgins Clark novels. The fact that a new novel was released every year in April, right around Dawn's birthday, made gift-giving easy and we loved the fact that the settings and characters seemed so real. And then we both read "Loves Music, Loves to Dance" and fell in love with the main character's love interest--such a handsome, supportive, loving guy!--only to discover in the end that he was the villain! I'll never forget Dawn's reaction: "I hate it when the author makes you really like a character and then he turns out to be the bad guy!" She was so disappointed that she almost swore off the MHC books. She didn't (and neither did I) but I learned that readers come to trust authors to deliver consistency with their books. Whether she received a slew of complaints or not, I'll never know, but I did notice that never again did Mary Higgins Clark disappoint us with another enormously appealing villain.
So that is one audience member I always wanted to deliver the goods to. I think she was always pleased with my work. In any event, when you write for your audience, you must be true to yourself and what you enjoy reading, That's how you find your true audience.