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The cover was TARDIS 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, teasingly 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; whenever River met him, she would name events she had experienced with him to see if he had experienced them yet. The Eleventh Doctor, in his personal future, explained the purpose of the diary to her; she had no clue what it was supposed to be for until then.
Upon saving the Eleventh Doctor's life (after killing him) by giving up her remaining regenerations, River ended up in the best hospital in the universe. It was there that the Doctor gave her the diary. This was done to make sure she kept track of their meetings to keep their timelines from becoming corrupted. (TV: Let's Kill Hitler)
The day after her admission to the hospital, River described that the only things she was left with were a bag of clothes with too many short skirts and her diary. Since pens were not readily available in the 52nd century, the nurses had to order her one from an antique shop so she could write in her journal. Tucked inside the diary she found a business card for Professor Arthur Candy from the Department of Antiquities at Luna University and numbers for a comlink code. (GAME: The Eternity Clock)
While River Song was studying at the Luna University, she did research on the Doctor, and learned a story he was going to die at 5:02pm on 22 April 2011, at Lake Silencio; she recorded this in her diary. (TV: Closing Time)
River brought her diary with her on a trip with the Eleventh Doctor shortly after she had been imprisoned in the Stormcage Containment Facility. The Doctor explained his rules for the diary: River was to use it to keep track of their meetings and adventures together. (HOMEVID: First Night)
In the 52nd century, in Stormcage, River was writing about one of her outings with the Doctor before taking a phone call from Winston Churchill and escaping to help her husband. (TV: The Pandorica Opens)
After the Eleventh Doctor reset the universe at the cost of his own erasure from existence, River temporarily gave Amy Pond her diary as a wedding present to bring him back. Later, the Doctor returned the diary to River, telling her that all the writing had returned, but he hadn't peeked; foreknowledge is dangerous. (TV: The Big Bang)
River spied on the Doctor's first nine known incarnations, learning what her husband looked like in his past to help her identify him for their future meetings. However, she ended up getting caught by his first incarnation before he left Earth with Ian and Barbara. Hearing Susan call for her grandfather, River left, not ready to know if she'll be the mother of one of Susan's parents; she had no clue if this Doctor was one she'd meet. She noted an opinion of each incarnation. (GAME: The Eternity Clock)
At some point or another, River also learned about the War Doctor and obtained a picture of him, (TV: The Husbands of River Song) as well as pictures of every other incarnation of the Doctor up to the end of his first regeneration cycle. (TV: The Time of Angels, The Husbands of River Song)
On returning from a trip with the Doctor for her birthday, River met Rory at Stormcage; he wondered if they'd met yet in her timeline, letting River know that he still didn't know she was his daughter. She checked her diary to be sure of where the Eleventh Doctor was in his timeline. By this point, she'd already been to Demons Run in her past and revealed her identity to the Doctor and her parents. (TV: A Good Man Goes to War)
In 2011, River and an 1103-year-old version of the Eleventh Doctor compared diaries. At this point in his time stream, the Doctor had a similar diary. By this point, she'd visited Easter Island and met Jim the Fish. She seemed fond of Jim when the Doctor mentioned him. (TV: The Impossible Astronaut)
After the crash of the Byzantium, Amy asked River what her diary was. The Eleventh Doctor, who had yet to experience many events recounted in it, told her to stay away from it; he called the diary "her past, my future" before going on to explain their timelines rarely intersect in the right order. (TV: The Time of Angels)
During her imprisonment, the Eleventh Doctor visited his wife and asked, for the first time, to break their rules and read her diary, because he was charged with war crimes he couldn't remember and he had to find out the truth. She said to have read it several times "from cover to cover" and to be sure he couldn't be guilty. After a first denial, River agreed and lent him the diary; eventually, he chose not to read it and hired her directly in his mission. (COMIC: The One)
By the time the Twelfth Doctor met River, River noted her diary was almost full and that the Doctor would know exactly how many pages she needed. It turned out to be providential, as in the middle of a dangerous scrape where River and the Doctor had tried to recover a priceless diamond, they noticed a familiar planet right before they ended up crash-landing on it: Darillium. Following the crash, Doctor decided to set in motion their long-delayed date to see the Singing Towers, a night lasting for twenty-four years which would be his very last encounter with her. (TV: The Husbands of River Song)
When the Tenth Doctor met River Song for the first time from his perspective, she attempted to "do diaries" with him, naming events which they had experienced together; he did not know who she was. When the Doctor tried to read the diary, River stopped him and told him that, by his own rules, he was not allowed to read it. (TV: Silence in the Library)
After the events in the Library and River's death, the Doctor left her diary in the biography section of the Library, now completely full. He and Donna Noble considered reading it to find out their futures, specifically Donna's (given River's reaction to her identity), but chose not to. By this point in River's timeline, the diary had seen quite a lot of action. Its pages had been heavily written in and weathered. The cover had also become worn, and the spine of the book was now cracked and peeled away slightly from the bindings. (TV: Forest of the Dead)
Nardole retrieved River's diary from the Library after her death and delivered it to the Twelfth Doctor after his night on Darillium ended. While confronting the Monks, a virtual version of the Doctor had the diary with him. (TV: Extremis)
In addition to adventures, River's diary also contained detailed information on alien creatures including handwritten notes, drawings, photographs, cutouts, and the names and events surrounding various people who have encountered these creatures. These entries include: Weeping Angels, Saturnynians, and Silurians. (WC: Monster File: Weeping Angels, Monster File: Vampires, Monster File: Homo Reptilia)
One diary entry contained notes for piloting the TARDIS, including a drawing of the console, and another was a to-do list prior to River's archaeological expedition of the Library. (GAME: The Eternity Clock)
River's diary has pictures of the Doctor's first twelve incarnations, including the War Doctor. However, she does not possess a picture of the Twelfth Doctor as she believed his regeneration cycle ended with the Eleventh Doctor. (TV: The Time of Angels, The Husbands of River Song)
She recorded several dates with the Doctor, notably a date with the Eleventh Doctor from an early point in her timeline, where he instructed her to begin using the diary. (HOMEVID: First Night) She also noted a trip she had with the Eleventh Doctor to Easter Island (TV: The Impossible Astronaut) a picnic at Asgard. (TV: Silence in the Library, The Husbands of River Song, PROSE: Picnic at Asgard) a trip with the Doctor to Florana to see the golden sunset, (AUDIO: Signs) and a visit to Manhattan. (TV: The Husbands of River Song)
Other notable events included rebooting the universe, (TV: The Big Bang) the Battle of Demons Run, (TV: A Good Man Goes to War) an encounter with Jim the Fish, (TV: The Impossible Astronaut, The Husbands of River Song, GAME: The Eternity Clock) the opening of the Pandorica. (TV: The Husbands of River Song) and the crash of the Byzantium. (TV: Silence in the Library, The Husbands of River Song)
River also recorded an adventure in the Bone Meadows. (TV: The Time of Angels) her second trip to Lake Silencio, 22 April 2011, at the Doctor's invitation,[statement unclear] (GAME: The Eternity Clock)
River recorded stopping the Eternity Clock from destroying or rewriting time. (GAME: The Eternity Clock) As an add on to that adventure, she included an entry (titled "An Unearthly Mess") in which she got caught by the First Doctor snooping around I.M. Foreman's junkyard in 1963. River also made several other less detailed entries about her encounters with earlier Doctors, from his second incarnation through to his ninth, not including the War Doctor. (GAME: The Eternity Clock)
Her final recorded adventure was that of a date with the Twelfth Doctor at the Singing Towers of Darillium. This was her last outing with the Doctor before she went to the Library where she met the Tenth Doctor who did not yet know her. (TV: The Husbands of River Song, Forest of the Dead, HOMEVID: Last Night)
Behind the scenes
As production on the 2012 video game The Eternity Clock game predates the War Doctor's debut in the 2013 television story The Name of the Doctor, the entries in River's diary involving past incarnations of the Doctor do not refer to him. Following the broadcast of The Name of the Doctor, in the 2015 television story The Husbands of River Song, it is revealed that River has met the War Doctor as well when his face is included among the photos of the Doctor she owns.
in an interview with alex kingston in the brilliant book 2011, she reveals the book is full of pictures of the doctor and places that river has visited