If I come across one more "adult" book in which someone finds a mysterious book of fairy tales that sets them on a path to unravel a mystery from the past that will change their life, I am going to SCREAM.
I feel like Mysterious Books of Fairy Tales have become the attic in the suitcase, the trope that keeps getting used over and over to propel and organize a protagonist's quest to discover The Truth About Her Past, Her Family and Herself. The writers who use this always feel compelled to include several of the fairytales, bracketed somehow in the story, and the fairytales are usually nowhere near as mysterious or interesting as the writer or protagonist think. John Connolly has some good fairytale usage in The Book of Lost Things, and Diane Setterfield uses them well also in The Thirteenth Tale. But in each the authors are working with at least partly extant fairytales, not inventing them wholesale (and gods, the inventions! dark forests, mysterious men, forlorn little girls, blah blah blah blah). Pan's Labyrinth, of course, also uses a book of fairytales to marvelous effect. And then, of course, there are The Tales of Beedle the Bard, which serve quite a good purpose and somehow fit into the book in which they first appear.
But most of the books I come across that include Mysterious Unpublished Fairy Tale Books are pretty thin. Often, I flip through them and decide against even checking them out of the library. I tried - I tried hard - with Kate Morton's Forgotten Garden, but it just screams with every cliche imaginable. I couldn't get through it.
It feels cheap, to me, for adult authors to keep returning to the well of Old Fairy Tales, over and over, as if just by referencing them, some important childhood touchstone is reached. Like suddenly the narrative acquires a mystery and profundity by dipping into the realm of Fairy Tale. When in fact, most fairytales (if you read Grimm's or Perrault's, anyway) are rather prosaic, in a way, highly moralistic and often rather grisly (see for example "The Juniper Tree" in any collection of Grimms' Fairy Tales). The child/childhood + fairytales = MYSTERY!!!!!!!!!!!!!! formula that these writers keep reaching for is just tired. It relies on some boring assumptions about both childhood and fairytales and in its way, is as bad as the nostalgic myth of childhood innocence. And fairytales are already so freighted with their own metaphorical and historical weight, adding a new layer into a new work of fiction often just tips the scales way too far. You need a very, very deft touch as an author to pull this off (Setterfield and Connolly both do this, I think), but it takes a very skilled writer to do so, and one with a real sense of the scope of literature - including that too-easily-ignored field of children's literature.
So - writers! find a new trope, and leave the fairy tales alone. Unless, of course, you happen to be Mother Goose herself.