I think the word that first comes to mind is "irreverent." The characters are delightfully goofy (except when they're not), and the breaking of the fourth wall early on and repeatedly seems to indicate this will be a rule-breaking book. In fact, it's only as things go on that it becomes very clear that, in fact, it's a highly rule-obeying book. It's just that those rules are a bit different.
I loved this book and got into it like a well-broken-in pair of jeans. In total fairness, I can see the stereotypical characters getting on the nerves of a different kind of reader. I can imagine someone becoming annoyed with the predictability of the action unspooling. Just as there have been books in this range where I was not the target audience, those would not be the target for this book. This time, I feel like I am.
I also enjoyed this much more than the Second Doctor adventure on which the concept was based, although I think that's largely down to the TV limited budget factor. Here we have a Land of Fiction that literally is only bounded by what its Master can be arsed to delineate. The revelation of who that Master is makes perfect sense, and it fits perfectly with the previous events in the book.
It wouldn't be one of my reviews without me talking about Ace, of course. Benny shines here, and it's one of the first times I really love her and can see her meriting the adulation. Here, though, the Doctor nearly reaps the whirlwind with Ace. He's lied straight out and by omission so many times that he nearly loses Ace's willingness to even work with him at all... And this time it's not his fault, but he has to deal with it anyway. It really makes me wonder where they can go from here.
I highly recommend this one, and if you're not the target audience, just remember: it's only a story...