Reading Harry Potter I came across this:
一人の男が現れた。あんまり突然、あんまりスーッと現れたので、地面から湧いて出たかと思えるぐらいだった
To me it seems to say "A man appeared. So quickly, so suddenly, that it could be thought he emerged from the ground."
What confuses me is 「あんまり」, which when I look it up, seems to me the same as the negative 「あまり」. However, wouldn't that then change the meaning to "not quickly, not suddenly"? Is 「あんまり」 not being used as a negative in this context?
あんまり is the emphatic version of あまり. あまり is a noun derived from verb 余【あま】る, "to be more than, to be extra".
In terms of usage, あまり is indeed generally used with negative verb forms -- however, the noun itself has no negative meaning.
Consider the English word "much", of vaguely similar meaning. On its own, it has no negative sense. But if I use it with a negative verb expression, it adds an additional layer to that negative sense.
Adding the あまり shifts the meaning from "I don't eat", to "I don't eat much".
あんまり突然【とつぜん】、あんまりスーッと現【あらわ】れたので、地面【じめん】から湧【わ】いて出【で】たかと思【おも】えるぐらいだった
This uses あまり in a positive way, and it emphasizes what happens in the first part, as the reason (that ので conjunction) for the second part.
In positive expressions using あまり, I'm more familiar with the phrasing あまりにも, but even there, I've found that it's often used to state that something has happened to such a degree, that this causes some other thing. The あまり is again used for its core meaning of something like "muchness, degree, extraness":
[VERB]s so much, that Thing B [VERB]s.