This research is a study of demands for books in library circulation systems. Demand data for random samples of books were collected and fitted to various standard distributions. The numbers of demands for collections of books are shown to be Negative Binomially distributed. As is shown, this implies that the numbers of demands for individual books in the collection are Poisson distrubuted and that the demand rate varies from book to book according to a Gamma distribution. Using these facts and assuming Exponentially distrubuted loan intervals, a model is developed which will predict the availability and unavailability of a book in a library. The practicality of using the model is demonstrated. (Author)