A new computer model is proving accurate in predicting how books will sell. “Information about a book travels through the network of potential buyers in two possible fashions: exogenous and endogenous. Exogenous shocks come from sources outside the system they affect, like billboards or newspaper articles; endogenous shocks are made up of very small exogenous shocks that happen in a coordinated fashion, like word-of-mouth recommendations. The model predicts how sales will decline after they peak according to how the peak occurred.”