I’m searching for a Book I forgot the name of

For her Birthday I want to gift my Girlfriend a Book she wants to read, she showed it to me but i forgot the full name..
I believe it was a green design and the name was " ____ Monster" or "Monster ___", so I'm missing the second word.
I think the theme was something like enemies to lovers, romantic genre and I believe he was a spy (?) or police agent or something..

I searched the Internet but like I thought there are many Books with the word Monster in it.
Obivously I don't wanna ask her, because I dont wanna ruin the surprise
So if anyone has an Idea, I'm thankful for any help!