This is easily something that entire PhD theses could be written about.

Which isn't my ambition (I already have one, and no desire to get more), but I could talk about a few trends that I noticed.

Goos point - there is certainly a lot to talk about here!

I would enjoy some material on the detectives who gathered all these tales.

Clemens Brentano and Achim von Arnim, who published Des Knaben Wunderhorn (The Boy's Magic Horn, 1805–1808), a major collection of German folk songs that actually inspired the Grimms to begin their own work.

Dorothea Viehmann, a storyteller from Hesse who contributed many tales directly to the Grimms — she was arguably one of their most important sources, though often uncredited.

The Wild family (particularly the sisters Dortchen and Gretchen), neighbors of the Grimms who told them numerous stories. Wilhelm eventually married Dortchen Wild.

Ludwig Bechstein, who later published his own Deutsches Märchenbuch (1845), which was actually more popular than the Grimms' collection for a time in the 19th century.

The Grimms also corresponded with folklore collectors across Europe, including the Scandinavian Peter Christen Asbjørnsen and Jørgen Moe, creating a whole network of folklore scholarship.