Truthful Newington is an heiress in an alternative universe Regency England. In this world, magic is everyday, with everyone having one ability or another - the heroine has some small weather making abilities and animals like her. Her maid is part-fay and can't touch iron(which must be disastrous in a world where iron is becoming a large part of the technology!). Napoleon is imprisoned, not on St Helena, but inside the Rock of Gibraltar. Truthful's mother owned the magical Newington Emerald, which is given to the women of the family. Nobody knows quite what it does, apart from affecting weather, but it's powerful. Very powerful. And on the day of her eighteenth birthday, when her father brings it out to show her and her three cousins, it disappears. Truthful's father takes to his bed and Truthful, disguised as a young man, must try to find it before the thief makes it do something terrible...
If you're expecting another Old Kingdom novel, you will be disappointed. This is a Regency romance with magic, as advertised on the cover. If it resembles anything, it's Gail Carriger's delightful Umbrella Protectorate novels, which started with Soulless. (And I see from the Goodreads reviews that Gail Carriger herself likes it). It is funny and exciting and has a number of delightfully over-the-top characters, such as Truthful's three boy cousins, and her great-aunt, Lady Badgery, who wears a fez and knows a lot more than you'd expect about male clothes. (The reason for that is given late in the novel). Lady Badgery is a skilled scryer who can also cast glamours to help Truthful pass as a boy when she needs to.
The love interest is not quite what or who he seems. There's a lot of humour as he tries to explain and is interrupted each time, soon after he finds out that the young man he has been with is a young woman(something to do with a moustache that has had a glamour put on it to enable Truthful to swap back and forth easily, which falls off when soaked...).
It's a frothy romp in which Mr Nix lets his hair down and plays. The novel was actually written back in the early 1990s, as part of a modern-day, never-published thriller set in a publishing company, before the author became famous for his Old Kingdom series, so in a way this is the original Garth Nix.
Read it, have fun and don't get annoyed if Sabriel doesn't wander through. You can always go back to the serious stuff later.
Available in bookstores and on-line from September 23rd.