This was probably my favorite book of the series, with a satisfactory ending.
I love the writing style of these books, there is a lightness to it that makes me smile all the time. 
This one has dirigibles, a boat trip, balloon trips, a cute meta-natural baby (Prudence), children traveling with their parents, Egypt's ancient mysteries, and adorable supernatural beings. (Vampires and werewolves). 
It's witty, sensible, and classy. It was a true feel-good read. And I felt compelled to continue reading the next series (The Custard Protocol) that focuses on Prudence as a grown-up lady.