Complex sentence: He also devoted a considerable amount of his time to studying hypnosis (which he used to help his wife with birth labor) and parapsychological phenomena, but a book he had written on these areas got lost during the Spanish Civil War.
Split into simpler sentences: He also devoted a considerable amount of his time to studying hypnosis (which he used to help his wife with birth labor) and parapsychological phenomena. A book he had written on these topics was lost during the Spanish Civil War.