The Secret Books of Saxon were a set of diaries written by the Master during his time as Harold Saxon. He had left them behind for a number of trusted disciples who followed him without question. They contained information necessary for bringing about his return if he was ever defeated or killed. They held a great many secrets of Harold Saxon, including his name being "the Master".
After the Master was killed by Lucy Saxon, his disciples acquired his ring and used the information within the Book of Saxon to bring about his return. To accomplish this feat, the Book of Saxon informed them that they would need the Potion of Life as well as biometric data from the Master. To retrieve this, the Governor took over the prison facility holding Lucy Saxon and took an impression of her lips, which they used to bring about the resurrection of the Master. (TV: The End of Time)