We can't tell you how many times we shook the pages of this book and screamed, "Just get a diary already, Herzog!"
Herzog's letters are definitely the central symbol in this book, since they pretty much address every theme in Herzog's life—disappointment, rejection, friendship, loneliness, you name it. On top of that, we can see a direct connection between the number of letters Moses writes and his mental health. The worse he feels, the more letters he writes.
At the start of the book, for example, we hear that:
Hidden in the country, he [Herzog] wrote endlessly, fanatically, to the newspapers, to people in public life, to friends and relatives and at last to the dead, his own obscure dead, and finally the famous dead. (1.2.)
The fact that he's writing to dead people suggests that Herzog is writing these letters more for his own sake than for the people he addresses the letters to… as well as suggesting that he might be a tad off his rocker. Herzog often loses track of time while writing his letters to nobody. Sometimes an entire day can go by before he realizes he hasn't looked up once from his desk:
He was astonished that a whole day had been spent scrawling a few letters. (5.73)
Hey, who among us hasn't lost entire days (especially post-breakup) to binge-watching Netflix and eating cold Chinese food? But somehow the fact that Herzog is detailing his heartbreak and mental disarray in writing makes it more severe, as well as more symbolic.
But despite the effort he puts into these letters, the narrator reminds us that he never mails any of these letters. Rather, the letters are conversations that Herzog is having with himself. And when he eventually recovers from his divorce to Madeleine, he no longer needs to write these imaginary letters.