A study in drowning by Ava Reid

Effy Sayre has always believed in fairy tales. She’s had no choice. Since childhood, she’s been haunted by visions of the Fairy King. She’s found solace only in the pages of Angharad - author Emrys Myrddin’s beloved epic about a mortal girl who falls in love with the Fairy King, and then destroys him.

Effy’s tattered, dog-eared copy is all that’s keeping her afloat through her stifling first term at Llyr’s prestigious architecture college. So when Myrddin’s family announces a contest to design the late author’s house, Effy feels certain this is her destiny.

But Hiraeth Manor is an impossible task: a musty, decrepit estate on the brink of crumbling into a hungry sea. And when Effy arrives, she finds she isn’t the only one who’s made a temporary home there. Preston Héloury, a stodgy young literature scholar, is studying Myrddin’s papers and is determined to prove her favorite author is a fraud.

As the two rival students investigate the reclusive author’s legacy, piecing together clues through his letters, books, and diaries, they discover that the house’s foundation isn’t the only thing that can’t be trusted. There are dark forces, both mortal and magical, conspiring against them - and the truth may bring them both to ruin.
 
- Goodreads Summary