Be prepared for one of the most exciting page-turning reads you'll ever encounter. This book will grip you from the first page and not let you go until the end. It's the longest of the four Sally Lockhart books, but you won't notice the length as the plot just keeps going and thickening all the time.
As if that wasn't enough, in between the excitement there's also a wealth of incidental detail about social conditions in Victorian England, and especially about the rights (or lack thereof) of women.
Please note: the four Lockhart books should be read in order. This book gives away a number of significant plot developments in the first two (Ruby in the Smoke and Shadow in the North), so you should read those first. They're also great, and you won't truly appreciate this one without having read those first.