I popped into Waterstone's today and on the way into the Costa's I went past the SF section and saw that Holly Black's Tithe, Valiant and Ironside have been republished in adult editions:
Synopsis: Sixteen-year-old Kaye finds a white-haired young man of exotic appearance bleeding to death in a rain-drenched wood and saves his life. From that moment nothing in her life will ever be the same. Soon she is plunged into an inhuman world of conflict and betrayal, with a sacrifice -- hers -- and a kingdom as the prize. Cleverly combining romance with the resonance of myth, Holly Black masterfully evokes the dark heart of her conjured realm where Kaye must fight for her lover and her survival -- a place of wonder, to be sure, but also of trickery, decadence, brutality and blood.
The adult edition on the left is £7.99 and the teenage edition on the right is £6.99 (before any discounts). I do like the adult covers but I cannot comment on how they reflect the content, as I've not read them (yet).