The Brothers Grimm, as is well known, did not collect fairy tales with the rigor of a modern folklorist. Many of their tellers were middle-class, not peasantry, and of Hugenot and so French descent. Furthermore, they seriously reworked them.
This is a study specifically of what they did to rework them, treating them as Kunstmärchen, literary fairy tales.
There's a wealth of information about it, especially since they talked about sources, and sometimes we have their original notes, and certainly, we always have the various editions of the tales. Given that they discuss Giovanni Basile's Cenerentola and add to the story between editions an incident where the father asks his stepdaughters and daughter what they want, and the daughter's gift is much less pricey, it would appear clear where they lifted it. Some are quite simple. When Hansel looks back and is ordered onward, he claims to be looking back to see his white cat, or white dove, and in the first edition, the mother says it's the dawn red -- afterwards, this was changed to the morning light glinting from the chimney, which would at least make the color plausible.
What I found truly fascinating was the instances of earlier version that he cites -- most, of course, literary. But they're still neat: a version of Snow White in which the stepmother has a talking dog that betrays Snow White again and again, and the dog's name is Mirror, or the rationalized version where the stepmother's problem is that the doctor compounding her poisons, under attacks of conscience, only makes a sleeping draught -- and the happy ending is that the doctor prospered after that. A play of Little Red Riding Hood where she tells her grandmother she will have to make her a new hood for her confirmation -- red hoods were common, because confirmations were often held on Pentecost, and they wore red in the honor of the feast -- but her grandmother thinks black more sober and suitable for church rather than a dancing floor, and in which the girl chaffs the hunter for wearing green. A Cinderella who arrives at the ball so finely dressed and with so many attendants that she looks like a prostitute arrested on a public thoroughfare, surrounded by police -- then, this is the one that murdered her first stepmother at the instigation of her governess, and then brought her second one on herself by persuading her father to marry the governess. The early medieval Snaefrid, who appears to be an anti-Snow White -- the king mourns over her body, which remains rosy, to the detriment of the realm, until a wise councilor persuades him to change the blankets and clothes she had, which promptly prove her corpse rotten, and emitting reptiles, so they burned it, and the king got better. How Perrault's title could mean either "Sleeping Beauty in the Wood" or "The Beauty In the Sleeping Wood." How Brynhild saved Little Briar Rose from exclusion, by proving it was a German tale.