A simple normative theory applicable to public library services was developed as a tool to aid libraries in answering the question: which books should be bought by the library? Although developed for normative purposes, the theory generates testable predictions. It is relevant to measuring benefits from services which are provided publicly because when "uncrowded" they have zero marginal cost, or because the market may be monopolistic. In effect, the theory uses part of the area under the demand curve for a book for the private good to approximate the area under the demand curve for the public good. Although formally applicable, the theory is not always helpful in the library sphere. (Author/SG)