This note contains spoilers for the book.

A suggestion of Tony von Krag's, and also conveniently available free from Baen.

This is a "Bordertown"-type book; definitely not the same universe, but investigating the interface of human and elf domains in the modern world. This is a more scientific model; magic is being studied in relation to quantum dynamics, and the elves are building mechanical flying gunboats. Pittsburgh has accidentally been trasferred (most of the time) from Earth to Elfhome, a planet a few hundred lightyears away, where the elves lived. There were natural gates before, small and not completely reliable. Also gates to at least one other planet.

I'm a bit skeptical of "NSA agents" with guns showing up to detain somebody; this is an alternate universe in various ways, but it's just not an NSA-like job. I'm sure the fact that the bad guys were the Oni was just a coincidence, wasn't it?

Somehow the structure isn't quite right here. The "shape" of the story doesn't really work that well. Tinker is pushed around too much, and learns far too slowly, so she ends up bouncing from calamity to calamity, being a victim, until she wins.

Still, Tinker is a pretty interesting character to be stuck in the head of, and her friends Oilcan and Lain are good too. Nathan is unfortunately believable. WindWolf is too naive for his age, though, and too ignorant of humans for his position as the viceroy of the area that Pittsburgh goes into.

What this actually reminds me of most is Aaron Allston's Doc Sidhe books.