Most of this stuff is already stored in one manner or another. Take a look at your gamelog.txt some time. Or browse legends mode. A journal would basically just need to parse stuff that's already there and put it in first person.
You could even have individual events get their own histories or journals. "The Invasion of the Plains Titan Sxanu Wolfwinters", for instance could be cobbled together, giving a brief description of what the Titan did beforehand, who the major combatants where, and how the battle played. There could be a "research book" job that scribes take up while they visit the scenes where the events happened and visit with survivors (or the families of the fallen) before "writing book". How the scholar chooses to describe the various things that happen could depend on how much he likes the dwarves involved (or their family members), what his favorite weapon is, and other stuff. If he likes, say, microline and, during his research, finds out that part of the fight took place on a microline bridge, he might choose to focus on how fine and cyan the bridge is (or was) rather than some other phase of the battle.
So, you could get something like:
The Wrath of Snaxu Strumpettinsel:
a telling of the invasion of the werewolf
written by Urist Rubyloved
The cover is made of badger leather. The cover illustration depicts the werewolf Snaxu Strumpettinsel. The spine is studded with octagon-cut rubies and menaces with spikes of tin. It is written sloppily, as though in a great rush.
"I, Urist Rubyloved, have studied the great invasion of the werewolf Snaxu Strumpettinsel and spoken with those who fought against him and Enhot Arcsmith, the widow of Urdim Matchedsticks. I dedicate this work to the late Urdim Matchedsticks, my close friend, may he be one with tin."
"The werewolf Snaxu Strumpettinsel ravaged many lands before arriving at Fallowfields, including Wetwood, where he killed eleven elves and three pigs, and Irontowers, where he slew six dwarves, five dogs, eleven chickens, and a turkey. He became a werewolf after being cursed by the god Narfl Blowfire after desecrating the temple Manyprongs in the year 17."
"His arrival at Fallowfields was noticed by the dog Rushfellow, the pet of the child Urist Silvertonge. Snaxu Strumpettinsel kicked the dog, driving the skull through the brain, and tearing the brain. Rushfellow was dead."
"Shortly thereafter, the squad Manyswords was ordered to kill Snaxu Strumpettinsel, among them Urdim Matchedsticks. The fight took place on a grassy field, stained with blood to this day. There is a microline wall nearby now, though it did not then exist. Urdim Matchedsticks attempted to punch the werewolf in the head, but his blow was parried. Snaxu Strumpettinsel bit Urdim Matchedsticks in the lower body, latching on firmly. Urdim Matchedsticks was unable to break the hold. Snaxu Strumpettinsel shook Urdim Matchedsticks around, tearing his lower body. Snaxu Strumpettinsel changed form into a Goblin. Snaxu Strumpettinsel released Urdim Matchedsticks."
"Later, Snaxu Strumpettinsel fled Fallowfields."
"Later, Urdim Matchedsticks perished of thirst."
The gist of it is that a werewolf killed a dog, mauled the first dwarf who ran off (underequipped) to fight it, transformed back, and ran away. Afterwards, nobody brought the injured dwarf a bucket of water, so he died of thirst. But, since his best friend was a scribe, this gets an epic retelling in book form.
I guess the main challenge here would be paring down and breaking up the reports into something reasonable. We don't want a million "X parried Y's blow with his shield". I suppose redundant entries could be rolled up it a single description, but doesn't help when there's twenty goblins fighting ten dwarves. Maybe pick out a half-dozen kills and roll up another half-dozen redundant entries, filtered by scholar bias. The scholar might prefer to write about the heroic doings of his friends and leaders and downplay his enemies' roles. If there's a weapon/armor involved that he likes the type of or made of a metal he likes, that could end up with it's own paragraph. All this is to keep the journals from being quite so repetitive and boring by injecting flavor text and essentially making every book a work of propaganda.