River Song's diary

River Song kept a diary over her adventures with the Doctor. The book itself 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 reasoning by 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 when River Song met him, in which she would name events she had experienced to see if that Doctor had experienced them yet. (DW: Silence in the Library/Forest of the Dead/The Pandorica Opens) 

History
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 Doctor, in his eleventh incarnation, reset the universe, River gave Amy her diary as a wedding present, though all the 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 to above event, told her to stay away and that it was her past and his future. (DW: The Time of Angels)

When the Doctor met River Song for the first time, at this point in his tenth incarnation, from his perspective, she attempted to "do diaries" with him, naming events which they had attended together. 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 with the Vashta Nerada on The Library, and River's death, the Doctor left the diary in the biography section of The Library. (DW: Forest of the Dead)