River Song's diary

River Song kept a diary of her adventures with the Doctor. The cover was blue and resembled the exterior of the Doctor's TARDIS. Since the diary was written from River's point of view, she didn't let the Doctor read it, jokingly saying "Spoilers" and often putting her finger on her lips. The book acted as an indicator of where the Doctor was in his personal timeline, for whenever River Song met him, she would name events she had had with the Doctor to see if that Doctor had experienced them yet. (DW: Silence in the Library, The Time of Angels, The Pandorica Opens)

History
In 2011, River and an 1103 year old incarnation of the Eleventh Doctor compared diaries. By this point, in his timestream, the Doctor also had a similar diary. (DW: The Impossible Astronaut)

In the 52nd century, in the Stormcage Containment Facility, River wrote in her diary before receiving a phone call from Winston Churchill and escaping. (DW: The Pandorica Opens)

After the eleventh incarnation of the Doctor reset the universe, River gave Amy her diary as a wedding present, though all its pages were empty. This helped Amy to remember the Doctor and bring him back. Later, the Doctor returned the diary to River, telling her that the writing had all returned, but that he had not peeked at it. (DW: The Big Bang)

During the events of the crash of the Byzantium, Amy asked River what her diary was, to which the Eleventh Doctor, who had yet to experience events recounted in it, told her to stay away from it. (DW: The Time of Angels)

When the Doctor met River Song for the first time from his perspective, in his tenth incarnation, she attempted to "do diaries" with him, naming events which they had attended together, but he did not know who she was. When the Doctor attempted to read the diary, River stopped him and told him that, by his own rules, he was not to read it. (DW: Silence in the Library) After the events in The Library and River's death, the Doctor left the diary in the biography section of the Library. He and Donna Noble considered reading it to find out their future, specificly Donna's (given River's reaction to her identity), but chose not to. (DW: Forest of the Dead)