Sorcery and Cecilia, Or, The Enchanted Chocolate Pot by Patricia Wrede and Caroline Stevermer
This book (and two sequels) are set in just post-Napoleonic War. Our little tagline might be something like "Jane Austen's World Meets the Society of Wizards with Nail-Biting Yet Hilarious Consequences." Both novels were written as actual letters sent from one author to the other. Sorcery and Cecilia consists of letters between two cousins used to carrying on adventures together. Cecy, at home in the country, begs for news of the city from her cousin Kate, who is coming out and having her first London Season. Kate starts out bored and awkward, until a witch mistakes her for a rival wizard and tries to poison her, using the enchanted chocolate pot of the title. Her hairpins still won’t stay in, but now she needs to find the other wizard to warn him. Meanwhile, someone keeps spying, very badly, on the social events that Cecy is attending. Should she report him or give him tips? The girls might be in over their heads, but at least they’re having a fabulous time. The writing is snappy in a nicely authentic-sounding way, the fashion impeccable and the characters intrepid.