A Tale Dark and Grimm (link leads to the review). So, I searched and found the second book - Through A Glass Grimmly. And it is as good as the first.
Just a note for everyone: if you were expecting a sequel (like me), well, expect to be disappointed. True, the characters are named Jack and Jill again, and they're royal, but apart from that, the storyline is completely different.
Again, the narrator takes us through various fairytales. And I'm super happy to note that Christina Rossetti's Goblin Market (link leads to download page at Project Guternberg), which was an awesome poem. Yes, it's a poem (but it's accessible and awesome).
To me, Jack and Jill undergo a lot more growth in this book compared to A Tale Dark and Grimm. In the previous book, Jack and Jill leave because they think they're parents don't love them (I...shalln't give you the spoilers), and so embark on their adventures. But in this book, true, Jack and Jill have issues with their parents (and Jack has issues with about every boy in the villange), but what prompts their adventures would be a quest. And I think because of the quest, there was much more character developement.
There is also one more main character - the talking frog, who serves as the ignored voice of reason. He's an awesome and likable frog though, and in the end, he's the one that enables Jack and Jill to complete their quest successfully.
Finally, the narrator. The narrator makes it very obvious that this isn't a saccharine sweet fairy-tale retelling. But, he also makes it obvious that this book is for kids. So it's not one of those "adult fairy-tale retellings" that seem to be the pre-requisite to turn anything dark, this book recognises that kids can handle dark and scary tales and gives it to them in spades.
I cannot reccomend this book enough. It's awesome(: