Further reading on the Borgias:
G.J. Meyer, The Borgias: http://amzn.to/2o2ZExE
Michael De La Bedoyere, The Meddlesome Friar and the Wayward Pope: Savonarola and Alexander VI: http://amzn.to/2oNEPmK
Further reading on papal history in general:
John Julius Norwich, Absolute Monarchs: A History of the Papacy: http://amzn.to/2oNKs4r
Michael Mullett, The Catholic Reformation: http://amzn.to/2pBq2fa
Geoffrey Barraclough, The Medieval Papacy: http://amzn.to/2oNAXST