Libraries vary greatly in size, but it is also widely believed that they vary in personality as well, the latter being manifested in their book buying patterns. But even if libraries had identical acquisitions profiles, the combination of size variation and random effects could give the impression of variation i n intention. We explore here the possibility that, for certain classes of books, this is the case. We define a probabilistic model of library acquisition, indicate how the model's parameters can be estimated, and test the mdel on a number of libraries for their collections in calculus.