The book covers the transition period between the Republic and the Imperial 
period using the technique of prosopography, which focuses on individual 
people, and their alliances.  A central theme throughout the book are the 
political parties of the major actors, and the major participants in each 
political party.
In brief, the transition between the Republic and the Imperial periods 
starts with Julius Caesar breaking with convention and marching on Rome, 
thereby seizing power.  Although J. Caesar wa a brilliant man, he was a 
one-man show, and failed to create a stable government which would outlast 
him.  When he was murdered, a period of chaos resulted due to the resulting 
power vacuum.  Caesar's adopted son, Octavian, eventually manages to 
reclaim his father's political party, and become the first true Emperor of 
Rome.
The book traces the development of the Julius Caesar's political party 
through patronage to his soldiers from the Gaul campaigns (in the Roman 
army, the deal was you served, and in return received direct cash booty and 
also ownership of enough land to farm), and also by the people Caesar chose 
to bring into his party, leading to long discussions of which families were 
granted the consulship in a given year.  By marching on Rome with his army, 
which was an institution of the people, not of the aristocracy (even though 
the aristocracy provided many of the officers), Caesar effected a social 
revolution, placing more power in the hands of the people than it had 
previously.
"The principes strove for prestige and power, but not to erect a despotic 
rule upon the ruins of the constitution, or to carry out a real revolution. 
 The constitution served the purposes of generals and demagogues well 
enough. When Pompeius (Pompey) returned from the East, he lacked the desire 
as well as the pretext to march on Rome; and Caesar did not conquer Gaul in 
the design of invading Italy with a great army to establish a military 
autocracy.  their ambitions and their rivalries might have been tolerated 
in a small city-state or in a Rome that was merely the head of an Italian 
confederation.  In the capital of the world, they were anachronistic and 
ruinous.  To the bloodless but violent usurpations of 70 and 59 B.C. the 
logical end was armed conflict and despotism.  As the soldiers were the 
proletariat of Italy, the revolution became social as well as political."
I personally found the book to be hard going in places. It is hard to keep 
the names straight, which is not helped by the Roman convention of naming 
daughters after the clan name, e.g. Julia for the Julians, generation after 
generation.  However, the language of the book is so delightfully different 
from the norm in computer science, and the topic manner is handled in such 
a rigorous way by grounding assertions about politics in the human 
composition of political parties, that I found the book to be well worth 
the effort.
- Jim