The Hard Facts of the Grimms’ Fairy Tales is a pretty good discussion of, not the origins of the tales in the Grimms’ collection, but in how the Grimms treated them and why. It looks at some of the publication history and the issues surrounding different editions, the changes in audience, and it deals with some pretty common interpretations of some of the tales (e.g. why Bluebeard is considered a cautionary tale about the evils of curiosity instead of, you know, the evils of killing your wives and butchering their dead bodies and then marrying again) and how they came about.
I guess it’s probably a bit dry if you’re not particularly interested in the topic, but I found it perfectly readable. It does help that I recently reread selections from the Grimms’ work, and looked at some of them in my SF/F class — I notice myself falling into some of the traps of thinking about these stories which Tatar discusses and evades — so that the whole topic is fresh in my mind and relevant to what I’m thinking about lately.
If you’re looking for salacious details of the “real” Bluebeard, or the real Hansel and Gretel, seek elsewhere. Tatar doesn’t really go in for that kind of interpretation of fairytale/folklore origins.