Interested in

[ambersnake]

I was reading Fool Moon by Jim Butcher and came across this sentence:

"But what I'm really interested in," I said, "is in knowing why your blood was in Marcone's restaurant

Comments

[kindigo]

I would say that's an error. Whether the mistake is the author's/editors' or Harry Dresden's is up to you. It's a spoken colloquialism is some parts of America, but it is, as you suspected, grammatically incorrect.

[ambersnake]

Ah, excellent. I was afraid it was yet another grammar rule I hadn't even heard of. Thank you!