SOLVED. fiction, set in 1800s era England, weird blue stuff [s]

[Nina]

I read the first book in the series about ten years ago. It's set in 1800s or thereabouts England, and there are three main characters: two males, one female. They are all trying to find out information about a secret society. I don't really remember much about them except that one of the males was a professional thief, I think, something like that.
The secret society had drugged them with this weird blue stuff, and they went on a train ride to sneak into the mansion where there was a party for the society. It was a strange book, quite long, but addicting, and I'd love to finish the series if anyone knows anything about them!

[Kit★]

Maybe The Glass Books of the Dream Eaters? There's 2 male characters and one female, and they did investigate this mysterious stuff, and there was a train ride, and a party at a mansion. It was definitely a long, strange book :)

[Nina]

That's it!!!! Thank you so much!