Based on the description, I was expecting this to be an activity book, but actually it's a junior novel (well, at 128 pages of large print, "novel" is really a stretch even with the "junior" qualification). The code wheel embedded in the cover is nothing but a gimmick to make the book seem more interactive than it is, since the code is always solved within the text of the book, though there are bonus activities in the back, one of which involves using the code wheel.
That said, I thought the book was cute. It wasn't as well written nor as illuminating with regard to the cartoon as its predecessor, Who in the World Is Carmen Sandiego, but it does serve as a nice "lost episode," seemingly set just after "The Fishy Doubloon Caper," that even explains the origin of Carmen's eyepatch from her Duchess disguise. Actually, I wonder if this plot was meant to be an episode that was then cut, perhaps explaining why there were 9 episodes in season 1, rather than 10 as expected.
At any rate, it reminded me a bit of the Carmen Sandiego Mystery series I read as a kid. It makes me glad that for as mindlessly destructive as Houghton Mifflin Harcourt has been to the Carmen franchise, at least they know how to put out tie-in books.