Open textbook initiatives have appeared as an alternative to traditional publishing. These initiatives for the production of alternatively copyrighted educational resources provide a way of sharing materials through the Web. While the open model of peer-produced materials enables the global reuse of textbooks, the combination of fragments to produce new textbooks tailored to particular needs is not easy. The heterogeneity of topics and target audiences, as well as the cultural and institutional differences of educational systems across the globe, raises the need for new compositional features, fostering--at the same time--the reuse of the newly produced combinations. This paper reviews the current models of reuse and sharing, and presents a quantitative analysis of the Connexions repository which provides evidence about the culture of reuse in open textbook initiatives.