King Lear and Sydney's Arcadia?

I received a request this evening from Hikari, who is studying at university in I assume Japan (from his .jp email address). Hikari asks:

I'm going to write an essay about King Lear's sub-plot and its source Sydney's "Arcadia"(the story of the Paphlagonian King). But I couldn't find secondary sources for my essay yet. Do you know any sources about it?

I had to go do my research to even understand the question, but I've never been shy to admit when I have no idea. The story of the Paphlagonian King refers to the Edmund/Edgar/Gloucester plot of Lear, which drew on Sydney's Arcadia as a source.

Some quick googling brought me here:

For the sub-plot of King Lear, Shakespeare relied upon a story from Arcadia, the epic romance by Sidney, published in 1590. We can see by examining Shakespeare's wonderful attention to the characters of Edgar and Gloucester, how much Sidney's tale of the king of Paphlagonia and his two sons sparked the Bard's own imagination.

I'm not quite sure what Hikari is looking for in his quest for "secondary sources". Does anybody else have some better sites to link?