Complex sentence: Cooper's work is held in many collections and institutions around the world, including the National Library of Australia and the New Guinea government, which purchased entire collections of his published works and commissioned from him two sets of postage stamps.
Split into simpler sentences: Cooper's work is held in many collections and institutions around the world, including the National Library of Australia. The Papua New Guinea government purchased entire collections of his published works and commissioned from him two sets of postage stamps.