Paranormalcy (Paranormalcy trilogy #1) by Kiersten White (HarperTeen hardcover, 31 August 2010).
Even though sixteen-year-old Evie has spent most of her life living in the headquarters of the International Paranormal Containment Agency, she considers herself a normal teenager. Sure, she neutralizes paranormal creatures around the world because she can see through their glamours, but she still loves clothes, watches the most popular teen soap Easton Heights, and her handler Raquel makes her study and do homework.
Her best friend Lish is a mermaid who also works for the IPCA, running the control center from inside a large aquarium. As expected, Evie is often lonely, at least until a paranormal named Lend gets caught trying to break into the building.
No one can figure out what Lend is, and Evie has never seen anything like him before. Bust she does learn that he's about her age, and lives as a human, going to high school and other "normal" stuff.
At the same time, there's paranormals are suddenly being murdered everywhere, and no one can figure out who. Some of the IPCA administrators think Lend might be involved, but Evie is certain he's not. And she's just as certain that the creepy faerie Reth is, even though Raquel says he can be controlled.
This story is a great combination of mystery, humor, romance, and of course the paranormal. It's been around a few years, but it's not at all dated.
The remaining books in the trilogy, Supernaturally (2011) and Endlessly (2012) continue the story of Evie and the IPCA after the identity of the killer has been determined and you'll want to read them right after you finish the first.
FTC Full Disclosure: I borrowed this book from my local library.