Atwater, Olivia. Regency faerie tales #3

Abigail Wilder's father may be the Lord Sorcier of England, but that does not mean that society is willing to accept her as a magician. When a dark lord of faerie threatens London, however, Abigail is determined to uncover the truth and save Lord Longshadow's victims - and neither good manners nor her father's worries will stop her. Abigail is not the only one investigating the terrible events in London, though. A street rat named Mercy soon insists on joining her cause - and while Mercy's own magic is strange and foreboding, she may well pose an even greater danger to Abigail's heart. - back cover