The Ancient Magus' Bride

Chise Hatori has been able to see things other people cannot since she was small, and the adults around her have been finding excuses to send her elsewhere for as long. She is fifteen when she sells herself on a London auction block, and a tall man with the head of a horned skull buys her — Elias Ainsworth, an ancient mage who introduces her to his cottage in the English countryside as his apprentice and, eventually, his bride. The series follows Chise's slow understanding that she is not, in fact, disposable, and Elias's even slower understanding of what he has agreed to.