"De qué se trata?"

Hola Kim

if you have an explicit subject, then you shouldn't say "se trata" but "trata".

El libro trata de un chico que....

And if you know you are talking about a specific book, even though the subject is missing from the sentence, you still say "trata de..."

For example:

You don't need se here.

You could have heard or seen it written, but I would say it's incorrect.

Saludos

Inma