The Letters of J.R.R. Tolkien
This book is not a "must read", but it is an enlightening read (best for high school and up). I have not read any other compilation of letters like this, so even the concept was new to me. There isn't a format, a thesis or an argument to unify the book. Rather, it is the life, work and times of Tolkien which generate the letters. He writes to his wife, his children, fans of his work, his publishers and various friends. The book is a subset of his letters (edited with the help of his son Christopher).