Very interesting and worthwhile -- won a Pulitzer. I might have found it difficult to read -- the audible version was abridged, and even then the first half was a bit dry, devoted to paleolithic plant ecology and the like. But then it 'heats' up, so to speak.
His basic question is why did Eurasia develop higher forms of civilization, and not Africa or the Americas or Oceania. He does not want to use (for good reason) biological explanations -- and probably finds (as I do) political/economic explanations (colonialism/exploitation/etc.) too facile -- and seeks to explain it by means of geographical and ecological factors... going back to the paleolithic period.
Basically -- domesticated food production began in the Fertile Crescent (Mesopotamia) -- and then independently in China -- and from there spread throughout Eurasia. Because plants are adapted to certain latitudes (length of day, seasons, etc.), the spread laterally and not longitudinally. Hence, cultivation spread to the Atlantic and to the Pacific but, once it cross the Mediterranean to North Africa, could not travel further south. The same holds for the domestication of animals -- and since pandemic germs derive from the Eurasians' close habitation with animals, who then developed immunities, they spread easily among the peoples of the New World, who had no such immunities. From domestication came settled life, urban life, highly developed forms of social organization, technology ( -- technology requires settled life, since nomads can't carry water-wheels on their back --) and... hence, guns.
It is a very reasonable argument -- and was worth the couple of hours of listening.
One striking example of the difficulty of longitudinal dispersion -- not just of plants, but of culture -- is that the Mayans had the wheel and the Incas (1200 km south) had domesticated llamas, and for thousands of years the twain never met.
I am not sure how I will feel about this book 6 months from now or 6 years from now -- so I will withhold judgment.