Stolen magic

Simon Malmain, chief enforcer of the Guardian Council, which oversees the use of magic in eighteenth-century Britain, is turned into a unicorn after a run-in with Lord Drayton, but he is luckily freed by Meg, a servant in Drayton's household, who then unites with Malmain in an attempt to keep the rogue mage from enacting a plan to keep the Industrial Age from happening.