I'm terrible at descriptions, so I'll let PW do it for me:
With Fate Conspire
Gifted storyteller and world-builder Brennan returns to the Onyx Court, a faery city that coexists with London, in her fourth historical fantasy (after 2010's A Star Shall Fall). As the Onyx Court is threatened by 19th-century advances in technology, the faeries and humans increasingly come into conflict. Eliza O'Malley is caught between the two worlds, both of which are often cruel and indifferent to her desperate search for her childhood friend, Owen, who was captured by the faeries seven years before. Unless Eliza can find Dead Rick, the dog-man who betrayed them, Owen will be lost to the faery kingdom forever. Series readers and fans of the Tam Lin myth will be captivated by this complex and vibrant depiction of a magical Victorian era.