There's a great interview to read about the backstory of A Tale Dark and Grimm at How an Elementary School Library Inspired Adam Gidwitz’s Debut Novel bBy Maryann Yin:
Q: What inspired you to write your book?
A: I was substituting at the library in my school. I was supposed to be reading a story to second and third graders, but I didn’t know what story to choose. So I picked up my thick, musty old book of Grimm’s Fairy Tales off the shelf and began to thumb through it. I stopped on a story called Faithful Johannes.
Now, I had read this story myself. I knew that it involved multiple close encounters with death, intrigue, betrayal, and a shocking happy ending. I knew that I had always wanted to read it to students. But I also knew that the happy ending involved two sweet, lovely children getting their heads cut off. So I didn’t think I could read it to these library students. And then I thought, what the heck? Let’s see what happens!
So I read it to the kids. And I kept stopping in the middle, to make sure they weren’t too scared, and I had to change some things, because the translation was pretty weird, and I added some funny bits, because making kids laugh is one of my favorite things to do in the world. When I finished—and the children’s heads had been cut off and put back on—the students stared at me. Their mouths were hanging open. You could [hear] a door, way down the hallway, creak and close. And then they went crazy. ‘That was amazing!’ ‘I can’t believe that just happened!’ ‘You’ve got to make that into a book!’ And so I did.
Q: Why did you pick the tales you chose to include in your book?
A: Well, I didn’t have a choice on the first one—because it’s Faithful Johannes, and the kids would have throttled me if I didn’t include it.
After that, I wanted to choose awesome stories that kids would not have read—Grimm is so chock-full of incredible tales that never get told. How many of these do you know: The Robber Bridegroom, The Juniper Tree, The Three Golden Hairs, The White Snake, The Black Dwarf, The Mouse, the Bird, and the Sausage. How many? Two? One? And they are SO much better
than Little Red Riding Hood. Why do we insist on telling the same six fairy tales over and over again to our children, when there are so many wonderful ones that would open their eyes and ignite their minds?
That’s how I chose my stories—tales that I knew would blow their minds. And, of course, I changed them. Most people are confused about the relationship between the stories in my book and the Grimm tales. About a third of the stories in my book are pretty faithful to the originals. Another third use elements of the originals but take sharp departures. And the final third are totally original.
There's more at the article so click through to read it all as usual...