This is the first book of a quartet following on with the adventures of Sophie Taylor and Lil Rose, the heroines of "The Sinclair's Mysteries". Two young detectives turn into secret agents for the next series of books. The stories are historically set in 1911 and the girls are on a mission with the Secret Service Bureau, where they travel to Paris.
The book has its own plot, but I believe that there is a running theme that will span all 4 books also. You will also have character appearances throughout from the sinclairs mysteries. So if you enjoyed those, you should enjoy this series and have some familiarity with some of the characters.
These books are suitable for confident readers from age 9+. The book has a few illustrated pages dotted throughout, just to add a little to the story, like an image of dossier papers, a poster or two and letters, which are integral to the story, which are to be read as part of the story.