Books received as specimen copies are received as gift from the publishers. As such it is nit advisable to record them in main Accession Register where "Purchased Books" are recorded. You should maintain a separate Accession Register and use a prefix [like G for GIFT] to put before the Accession number assigned to them By this way, you can separate purchased book bearing Accession No. 100 from the gifted book bearing Accession No. G100.
Hope I could explain the issue. For further details go through my reply at Ms. Mala A's post.