How Many Friends Does One Person Need?: Dunbars Number and Other Evoluti...
Great read for those interested in our human behavior and human evolution by one of the though leaders in the area. Full of tons of useful info and interesting facts that make you stop and think.
In response to request to "tell me more": The book is written by Robin Dunbar, who's famous for identifying "Dunbar's Number" - the cognitive limit on the number of people with whom humans are capable of maintaining close relationships. The definition is 'close relationships' is the type of person who, if you saw them in an airport, you would go up and say hello to (or would feel badly if you didn't). The number is generally accepted to be 150 for most, but ranges from 100 to 250.
So the book covers this topic in greater depth (breaking down different levels of human relationships and the cognitive limit for each), and then explores a ton of other interesting insights on human evolution. The content is somewhat academic, but it's still a quick read and easy to skim the parts you find less interesting.