The Life of Cesare Borgia by Rafael Sabatini, Biography & Autobiography, Historical

A condottiero, nobleman, politician and cardinal, whose fight for power was a major inspiration for The Prince by Machiavelli. Most people consider Cesare Borgia one of history's great question marks.
After initially entering the church and becoming a cardinal on his father's election to the Papacy, he became the first person to resign a cardinalcy after the death of his brother in 1498. His father set him up as a prince with territory carved from the Papal States, but after his father's death he was unable to retain power for long. According to Machiavelli this was due to his planning for all possibilities but his own illness.