You are right David. In Spain you wouldn't say
el libro se trata de política but
el libro trata de política.
However, it's very common the expression
¿De qué se trata? Meaning
what do you want to tell me? But this is a very specific use of this verb, only with this expression:
- Tengo algo que decirte.
- Dime, ¿de qué se trata?
Better than a reflexive verb, I would say that
tratarse is a pronominal one, since it doesn't convey any meaning of reflexiveness.