Imagine for a moment you are applying to be an extra in "The Hobbit"
The casting director asks "what makes you special?"
Your answer "I'm the only person available for the job."
The casting director looks out the window to see this:
People lined up to apply for the job.
The casting director says "NEXT!" and you're out the door wondering what the hell happened.
I mention this because all too often I'm seeing queries from writers saying there aren't any/enough books on their subject.
Since one of the things I look for are holes in the market, I turn to Amazon and search for books on that subject.
Too many times the search turns up more than 100 books. Obviously not all are good matches. But you don't need to find 100 to know that "I'm the only one" isn't on the right side of the truthiness scale.
What does this mean for you?
It means Know Your Field. If you want to write a picture book that reinforces a certain concept, you better have read every picture book in your library, and all the ones your librarian tells you are a good match for that idea.
That way, your answer to "what makes you special" is not "I'm the only one" but "I do this better than Title X" or "my book is more current than Y."
If you want to write a novel about world war two spies, you'd better know who Alan Furst is. And David Downing. And Ken Follet.
If you tell me there aren't any good books about being a cop, I'm going to bop you on the noggin with a copy of Edward Conlon's BLUE BLOOD.
Every single time you write "there aren't any books about this" I double check. EVERY TIME. If you get this wrong, it's game over.
How do you get knowledgeable about your area or topic? You read. A lot. If you haven't read at least 100 books in your area, you're not ready to start writing. This obviously is an on-going effort and keeping a reading journal or list is a good idea. I maintain a list of the published, non-client books I've read on Library Thing.
Read the books that are reviewed in PW, or the magazines that serve your genre. I subscribe to Crimespree, Mystery Scene and several others just to keep track of what's out there.
Almost all publishers have their catalogs online now. Go to their websites and download them, and see what they're publishing that you've never heard of. Read those. Keep notes.
And if you think this is a waste of time, let me remind you of this: one of the keenest readers of genre fiction is a guy named Lee Child. Heard of him? Before he was a writer, he was a reader. When he sat down to write his first book, he knew a LOT about what was out there, what worked, what didn't and most important, what he wanted to write about.
I saw this firsthand at Bouchercon three years ago when I walked through the book dealers's room with him. He knew dozens and dozens of authors and books. He'd read them and had opinions on them.
I vowed then and there to make sure I kept up on my reading. It's part of the job.