Reticence

The final book in the series alternates between Percy and the Spotted Custard's newly acquired doctor as the viewpoint characters - the latter turning out to be the daughter of a character previously seen in the Finishing School and Delightfully Deadly books. The budding romance between the two is a major element of the book, although actually rather less so than similar elements in the previous two books. Having said which, given Percy's nature, it does provide a fair amount of the humour in the book.

That it's the last in the series (at least, chronologically) is made apparent by how much of the first section is taken up by bringing back a number of characters from books set earlier on and seeing how they have fared. It's enjoyable seeing some of them return, and, for that matter, tying The Custard Protocol more closely in with the Finishing School YA books. Some of it leaves as many new questions as answers, which may well be dealt with in some of the planned spin-off novellas.

Once we get into the actual plot it's off to Japan where we encounter kitsune and more besides. This world's version of Edo is brilliantly realised, as a wondrous piece of supernatural steampunk, and there's some inventive worldbuilding elsewhere in the country as well. Some of this does mean that the plot is pushed into the background, and I'd argue that it isn't one of the strongest books in the series as a result, but it's a celebration of the setting and its characters, and, seen in that light, it's quite a success.