The Ninth Dragon Shifter by X. Aratare

Eight dragon shifters rule the world, but then ordinary human Caden Bryce becomes the ninth, destined to be a mate to one of the eight. Yet all Caden wants is his old life back.
Dragon King Valerius, most powerful of all the dragon shifters, is none too pleased to find another dragon in his territory. But when the other dragon shifters come to court Caden, Valerius finds that he wants Caden as his own.

WARNING: This is a long serial that is divided into book-length segments. Every single one ends in a cliffhanger. We anticipate there being 9 volumes.