This book was published earlier in the UK under the title The Popes : A History. That is precisely what it is. Not all 265 popes are mentioned nor can be in this survey of the papacy. But all of the major players are here.
What Absolute Monarchs does best is to really show what the job of Pope was before the 20th century. The Pope had to be mediator between squabbling aristocratic families, a negotiator of treaties between countries, the civic manager of Rome, and the defender of the faith. It was the rare man who could do well at all of the functions. There are very holy men who do wonderful things for the poor of Rome but are hopelessly inadequate to negotiate treaties or mediate. Those responsibilities take a diplomat or statesmen to maneuver around all of the politics of the day.
The Pope was in charge of Rome and keeping its commerce rolling as well as collecting tithes and indulgences, therefore the position of Pope could be very lucrative. So it becomes clearer how we wind up with the Borgias or the de Medicis who were less interested in the defender of the faith elements of the position.
Because the position of Pope was so tied up with the politics of the day, parts of the book can seem very dense. Arguing with the Emperor of the Holy Roman Empire and having to quell dissension in France, or disputes in Spain, reads more like international history or political history. But the Pope was very involved in all aspects of politics and international negotiations. It wasn't until Italy was united in the 19th century that the position of Pope becomes less political and more spiritual leader.
The author continues his survey of Popes to the present day. He has opinions on some of the conspiracies of the papacy, Pope Joan, death/murder of John Paul I and the rules of the antipopes. But the real benefit is in showing why some men were drawn to the position and others refused. The modern papacy is very different from what went before.