Memory Logs are books/journals that are used for by telepaths to project their memories, usually in pictures. Sophie owns a thick teal one with a moonlark on it. She was supposed to share it with Alden, the man who gave it to her, and let the Council know of anything suspicious; however, it is not known whether everyone assigned a memory log has to do this.
Sophie has also used her memory log to record dreams and important information throughout the books. It works especially well for her because of her photographic memory.
During the events of Legacy, Keefe was noted to have multiple memory logs that were not projections of his memories, but rather sketched-out scenes that he drew.