For fans of Regency romance--especially Regency with some magic thrown in, ala Caroline Stevermer and Patricia Wrede's Sorcery and Cecilia--this is a fun little romp of a book. However, Newt's Emerald is very different from Garth Nix's other books, so I think it's important to know that going in!
Truthful Newington is looking forward to her upcoming season in London, when the heirloom Newtington Emerald is stolen from her home. Desperate to get it back, Truthful finds herself in London earlier as expected, and forced to disguise herself as a boy (with some magical help from her great-aunt), since many of the places she needs to go in search of the emerald are closed to her as a boy.
There wasn't a whole lot of substance to the story, or to Truthful's growing romance (with, of course, the gentleman who disapproves of her female self but is helping her, disguised as a boy, to recover the emerald). But it was a lot of fun, and it's clear to see Nix's fondness for Georgette Heyer's books in this story. A quick, enjoyable read for fans of Heyer and other magical regencies.