This is a mystery/adventure fantasy novel in an urban setting that reminded me of an 1800s London or New York. I wouldn't call it steampunk or swords & sorcery but it has elements of both. The story is fairy fast-paced and there isn't a ton of the world-building you often see in the fantasy epics (religion is only lightly touched upon and magic is mostly in the hands of the enemies so it's not a big part of the story), but I didn't mind. The characters were what made this an enjoyable read for me. I was pleased when I read the epilogue because it was clearly setting things up for future novels with these guys (this works fine as a standalone story though).
Amaranthe, the main character, started out as something of a goody-two-shoes, quoting regulations at the criminals she was arresting, but she grew a lot as the story went along, and she was the one leading the team and coming up with the schemes in the end. I liked that she was a strong female character but not boringly so (no uber skills that let her hack down legions of soldiers single-handedly). She can handle herself in a fight, but she's just as likely to scheme her way out of trouble.
Sicarius, the other main character, is everything you'd expect from an assassin, but he felt quite original as well, and I was even more intrigued by the mystery surrounding him than the main plot mysteries. One of the big questions about him does get answered by the end of the book (I was close with my guess, but didn't get it quite right), but it left me with new questions that better be answered in following books! The author did a nice job making me like him and sympathize with him even though he's arguably still more of a villain than a hero in the end.
The relationship between these two was what I really enjoyed. I have to admit I was hoping for a kiss by the end, but if things are going that direction, it'll have to be in a future book.