Book one in an interconnected standalone in the Fae Kings of Eden series. While the books do not have to be read in order, it is recommended. Viciously Yours is a new adult fantasy romance short novel of about 60k words/250 pages. It is fast-paced and focuses solely on the relationship between the two main characters.
 
'I don't need a line to know what is considered good or bad, but if I did, there isn't a line I wouldn't cross for you. When Rennick was thirteen, the gods bestowed upon him the name of his fated mate--a human girl.
 
As the sole heir to the Mountain Kingdom, magic bound him to his kingdom until he ascended the throne on his twenty-fifth birthday, and as a human, she couldn't cross the magical barrier separating the humans from the fae. Heartsore and consumed with thoughts of her, he did the only thing he could and sent her anonymous letters and gifts... Until he crossed the barrier into her kingdom to claim her as his own.
 
Six months after Amelia's thirteenth birthday, she received a strange letter from a fae boy claiming her as his mate. Humans weren't taught much about the fae, and one look at the horrifying doll meant to be her belated birthday gift solidified her suspicions that someone was playing a cruel joke. Except the letters and odd gifts kept coming. Against her better judgment, Amelia fell in love with her mysterious admirer throughout the years, certain they would never meet. But on her twenty-fifth birthday, her walk to work was interrupted by two familiar words she'd read a million times. 'Hello, love.'
 
The male main character is obsessed with his mate and does a few morally black things to those he deems a threat to his relationship. There are no shades of grey.