But, I realize you're looking for an index of folk tales.

I found the Aarne–Thompson–Uther Index; The King of Love is type 425A, but that has more to do with motifs than their history.

You can see the ATU guide online here: https://libraryguides.missouri.edu/c.php?g=651166

Additionally, there is the Thompson motif index

The Collected Sicilian Folk and Fairy Tales of Giuseppe Pitré edited by Jack Zipes, Joseph Russo looks like a good start but does not get extremely detailed in its origins.

Unfortunately, I'm not finding much better than that in the form of an origins book