This is a very academic study (Princeton University Press 2011) of a very non-academic subject and the overall effect is a bit like Brechtian estrangement (Verfremdung): instead of portraits of godfathers who are, at the same time, chilling and charismatic, Varese offers correlations and statistical significance.
He works with a narrow definition of mafias as criminal organisations which offer protection (often willingly sought) backed up with the threat of violence. This makes mafias alternatives to the state - the organisation which in a given territory is able to claim a legitimate monopoly over the use of force to secure, when needed, life and property.
This contrast between private and public enforcement makes less surprising Varese's conclusion that mafias emerge and achieve success where there is a deficit of state power - where state organs are unable to protect markets and enforce debts and private groups step in to do so.
State organs may themselves operate like mafias providing "protection umbrellas" in return for bribes and retainers. This is what happens in contemporary China and once happened in "Tammany Hall" New York, making it hard for private mafias to break into the market.
On Varese's definition, the involvement of mafias in illegal rackets - alcohol, drugs, gambling, prostitution - is secondary to their main activity.
All mafia activities, if even half-way successful, generate large amounts of money and the most serious internal mafia disputes, often fatal for participants, seem to arise from free-lancing with community funds or even outright embezzlement. Varese documents this in his studies of Russian mafias.
As the title of his book indicates, his specific focus is on mafia mobility. He concludes that mafias are very much linked to a territory (just like ordinary state authorities) where they know everyone who matters, who can be trusted and who can't. They do not migrate voluntarily, only to escape state authorities or rival mobs. And when they do migrate, they are not always successful in establishing themselves in a new territory. The sub-title of the book is a bit misleading: "How Organised Crime Conquers New Territories". Varese's conclusion is that quite often, they don't - which is less sensational than the sub-title implies.
If you can bear the prose, Varese's book is interesting and his field research demonstrates personal courage.
But Varese does rather confirm a feeling I have that, nowadays, much of the best research we have is done not by academics but by serious investigative journalists. In my own recent reading, I would single out Barbara Demick's, Nothing to Envy: Real Lives in North Korea (Granta 2010) as a book which provides a mass of data in the context of a narrative at once sophisticated and compelling. It ought to be possible to write many books about the world's many mafias which achive that combination.