Lorelei Bell 
Spell of the Dark Castle [EPUB ebook] 

After Zofia and her family return to their world, everything should be fine… but is not. Zofia’s first assignment as a Knight is to go to Dark Castle under cover as an Ugwump — mortal — to spy on the owner, Saint Germain, who is believed to be a vampire.
On her trip to Dark Castle, Zofia meets Myron; a vampire with many secrets. After they encounter the handsome Saint Germain, Zofia uncovers truths about his past… and about their attraction for each other.