If you want to gain insight into Nietzsche's thinking outside of his usual philosophical writings, or follow his chain of thought throughtout his life, this collection of letters is somewhat helpful, but he does not seem to engage in the manner in which he does in his formal philosophical works. One of the features I found surprising in his letters is the courtesy he showed to his recipients. It is evident that Nietzsche treasured the friendships he had, and this is very apparent in his letters. And interestingly, I did not find any hostility in any of the letters addressed to Richard Wagner, considering the history of their relationship.
The book is well-edited, and there is an index of recipients near the end of the book. The editor also includes a general index with subentries that allow the reader to scan an entire topic. This is a helpful aid for amateur readers of Nietzsche, such as myself, but could also be helpful I think to dedicated scholors of Nietzsche.
I was only disappointed that more letters did not address more of Nietzsche's thinking on Dionysus and Apollo. It would have been interesting to read what he had to say about them via the "freestyle" of letter writing. Nietzsche's philosophical writings are actually the most frank and unrestrained of all in nineteenth-century philosophy. He is very honest with himself, and because of this he might be viewed as somewhat narcisstic by some readers. This may be true to some degree, but Nietzsche is refreshing in his style of writing, and actually it is quite entertaining to randomly move through his books and read his maxims and opinions.
The most interesting letter is the one addressed to Carl von Gersdorff on April 6, 1867. He is writing about what he has called "the scholarly forms of disease", and tells of a story about a talented young man who enters the university to obtain a doctorate. He puts together a thesis he has been working on for years, submits it to the philosophical faculty. One rejects the work on the grounds that it advances views that are not taught there. The other states that the work is contrary to common sense and is paradoxical. His thesis is therefore rejected, and he does not therefore earn his doctorate. Nietzsche describes the "not humble enough to hear the voice of wisdom" in their negative judgment of his results. Further, the young man is "reckless enough", in Nietzsche's view, to believe that the faculty "lacks the faculty for philosophy. Nietzsche uses this story to emphasize the virtue of independence: "one cannot go one's own way independently enough. Truth seldom dwells where people have built temples for it and have ordained priests. We ourselves have to suffer for good or foolish things we do, nor those who give us the good or the foolish advice. Let us at least be allowed the pleasure of committing follies on our own initiative. There is no general recipe for how one man is to be helped. One must be one's own physician but at the same gather the medical experience at one's own cost. We really think too little about our own well-being; our egoism is not clever enough, our intellect not egoistic enough."