Didactic is one of those words that sounds a lot more complex than it is. Simply put, it means trying to teach you something. Your teachers are didactic when they're trying to get you to understand something, and stories are didactic when they seem to be working pretty hard to get a message across. Sometimes that can be annoying, but in The Mysterious Benedict Society, we don't find the didactic tone too hard to take. In fact it's kind of fun, which is why we think it's didactic in a grandfatherly kind of way.
See, even though there are some pretty obvious messages here—propaganda is everywhere, you need to be a careful consumer of media, books are important, children deserve respect, there are lots of ways to be smart—we never feel like Trenton Lee Stewart is trying to cram anything down our throats. We enjoy the story the same way we might enjoy a tale told by a favorite grandparent—and while Pop-Pop wants the best for you, he also likes to see you smile, so he's not going to push his agenda too hard. Heck, he might even let you have candy at breakfast once in a while.