An article from the Catholic Encyclopedia. It is an old article but it does give one an idea of the theology behind the feast and the particular circumstances that brought it about. For those who know Italian, here is a link to an interesting article.
My take on Pope Urban IV's bull. It contains a link to the relevant section in Denzinger's online latin site. What I did in the article is to show the places in the document where the Scriptures is cited or alluded to.
An article from Otium Sanctum about the Gospel Reading for the feast in Year A. What I did was simply to show how certain key points in the discourse are re-echoed in the Gospel of John.