"What were they wearing?"
My wife notices clothing. I don't. When I try to remember a situation, the clothing data just isn't there. It's hazy, like trying to read a book in a dream. Shirts? Pants? A coat? Sure, why not. If I have to visualize specific garments, my brain fills them in, but the outfit's an invention, not a recollection. That's just how I'm wired.
My wife, in contrast, notices and remembers every hue, shade, zipper, and seam. More important, she knows what it all means. What each decision (or lack thereof) signifies about the wearer. This is why she's so good at creative direction. She got into that work sideways from illustration. She'd never have done so if she'd assumed everyone else had the same eye for sartorial details. Luckily, she recognized something: what was obvious to her might be valuable to others. (Maybe I inspired her: "If he's that clueless, maybe...")
If you're wondering what to write about, or if you're wondering what your book on X can do that the other five books already published on X could not, drill into the stuff you can't help noticing. Guaranteed, someone else wishes they could see what stands out so plainly to you. There's your first reader. Even if other people can see the same things you see, they probably can't articulate it the way you can. For instance, I've always liked Star Trek IV, but Darren Franich told me why. That's valuable to me. I'm sure you can think of similar examples.
Whatever you already know, whatever you already think, put it into words and see what happens. Instead of reaching fruitlessly for some ineffable insight just out of reach, tell us in plain English what's as plain as the nose on your face.