No, he read Kemmler's how to book, he has the talent to pull it off, as apparently most wizards do. That's why one of the Seven Laws is against it. However raising Sue once doesn't make him a necromancer. Since he didn't raise Mort from the dead, it wasn't necromancy.
I think there's a bit of a miscommunication, a differing definition, around the term "necromancer."
It seems to me that @Mira is using it as a functional/behavioral descriptor: someone who summons/binds the dead (corpses and/or spirits), someone who regularly does so. Is that correct, @Mira?
On the other hand, @ConspiracyTheorist seems to be working more on the basis of it being something inborn: a natural proclivity or talent, regardless of whether said talent is actively pursued. Is that correct, @CT?
Personally -- if my understanding is correct -- I think both are correct: Harry
DOES have an exceptional gift in this regard, substantially beyond most wizards' ability. Harry does
NOT routinely study or practice this, and only uses it rarely.
But Sue wasn't Harry's first act of necromancy; he had another stand-out instance: defeating Bianca, by summoning a vast number of shades. Notwithstanding the "weakened barriers" making that easier, we have to note that Harry:
- Did use necromancy
- Used it on an impressive scale
- Used it to defeat the student (Bianca) of a necromancer (Mavra)
- Failed to defeat Bianca using his own favored magical specialties (which usually are effective)
- Did all of that without formal necromantic specialization, just raw will & magic and... native talent