Since I don't know the exact 'nasty little letters true story', it could be a story of mystery. The 'nasty little letters' might have been discovered in an old attic and turned out to be part of a long - forgotten true event. They could hold the key to solving a family mystery or be related to a historical event that was covered up, and now the discovery of these letters is bringing the truth to light.
One possible example could be a story where a student was getting 'nasty little letters' at school. These letters were true in the sense that they were really being sent to the student. They contained mean comments about the student's appearance and intelligence. It turned out that a jealous classmate was sending them. The student had to find the courage to confront the situation and put an end to the bullying that was being carried out through these letters.
It depends. Sometimes a story labeled as such might draw inspiration from real events but have fictional elements added. So, it might not be a purely true account.
It isn't a true story. Usually, works like this are crafted to engage the readers' imagination and provide an interesting fictional narrative rather than representing real-life events.