The torn diary relates the last thoughts of a zealous priest, who has been jumping in joy thinking that Judgement Day has finally come, the N-4 Virus being the tool of God's wrath.
As the diary entry continues, however, the priest becomes gripped with horror, for when his congregation arrives at the church, he discovers too late that they are all infected, as they tear apart the priest's wife when she goes to open the church doors for them. Having realized that N-4 virus is not a divine act, and unable to escape the church, the priest laments his fate, asking why God has abandoned him. His writing grows more and more panicked as he hears the undead members of his congregation drawing near, before it finally streaks off the page (presumably indicating that the infected finally got him).
The Tattered Pages story tells about Jonathan Milman, a former officer of the Fairview Police Department who had witnessed the outbreak when he and his partner rushed to the Secronom building after hearing about the riot occurring there. He came far too late though—people were already infected, turning into zombies and proceeding to tear their living companions apart. On page 3 he tells of a fight that broke out amongst the group of survivors that had joined him in a barricaded library. The fight ultimately ended with half of them dead and several more wounded. Shortly after, some of the survivors broke down the barricades of the library in an attempt to escape, and zombies swarmed in, thus leading to his present situation. With the 3 other remaining survivors, he plans to leave the library in search of more survivors.
The Old Notebook story tells about another survivor who was bitten by a zombie while he was looting an old convenience store (some people speculate that it might be Johnathan Milman). His friends didn't have the heart to kill him, so they locked him in a side room instead. Through the first three pages, he tries to keep a conscious mind by writing about anything he can think of, until he finally succumbs halfway through page 3.
However, the 4th page is found to be covered with crude sketches. The word "Hungry" is written in barely legible letters at the top of the page, with another word (possibly "Grrr") written below it. There is also a crude humanoid figure drawn on the lower left portion of the page, and one can easily make out the word "Hello?" written next to it. (Whether the humanoid figure is human, zombie, or mutant is open to interpretation.)
Surprisingly, in page 5, the writer actually starts writing normally again, and expresses surprise that he's still alive before resolving to get himself out of the side room. This possibly shows that people who are infected can, somehow, overcome the state of infection and regain consciousness as a sane human.
The Old Notebook is perhaps the most insightful of the three lootable diaries, as it gives the reader some clues as to how the N4 virus infects humans. Based on the events related in the Notebook, zombified individuals appear to keep some measure of intelligence for a while even after succumbing. It is possible that their digestive system cannot digest normal food, and by one means or another they sate their hunger with human flesh as a result. Alternatively, it may be that if they don't feed on human flesh for a period of time, then the N4 virus will become attenuated and they will turn human again.
(In other words, it is possible that the only reason the zombies are roaming the streets in their current state is because they feasted on human flesh.)
It could also be speculated that some individuals may have a natural immunity or resistance to the virus. This is re-enforced by the fact that the Guards at the outposts sometimes say: "Sometimes I wish I wasn't immune, at least they get to eat every now and again." This means that some or all of the survivors are aware of their immunity to N4. This also doubles up as a possible reason we 'respawn'. (i.e. Because we don't end up zombies.)
However, nothing can be confirmed. As it has no real effect on the game, not much thought is put into it.