As a reader for a literary agency, I read lots of manuscripts of novels and memoirs that would like to become books. As a book reviewer for national magazines, I read lots of advanced review copies (ARCs) of novels and memoirs that are about to become books. How does a manuscript get from the first category to the second? What makes an agent, then an editor, then a publisher think, Yes, I can sell this?
It's not enough to be well-written, with flawless sentences, believable and intriguing characters, interesting settings, twisting plots, strong pacing, vivid scenes, a structure that makes sense, etc. Many manuscripts have all these qualities and more; their mechanics are perfect; they read like real books. But they read like books I've already read or don't really feel compelled to read. Supposedly every story under the sun has already been told, yet every month publishers release captivating novels with new takes on the old human story. My advice to manuscripts that want to become books is to reach beyond the competent and the familiar, to find your unique self. Maybe you tell the story from an unexpected point of view, or place it in an unlikely setting, or probe more deeply into the forces at play, or twist the plot in the direction opposite from where it was headed.
It's hard to get a first book published; with so many writers submitting manuscripts, the numbers are against you. And then there's luck, happenstance, connections, and the vagaries of the marketplace: things no one can predict or control. So give yourself whatever boost you can in the writing, and make sure your story sits apart from the other books you've read. I don't believe it must be great in order to sell; if it's good and it's different, I think it has a shot.