It's all well and good to read biographies of famous composers like Mozart or Beethoven. A good biography can give you a new perspective on the composer's life and a better understanding of their music. But for a truly rich understanding of a composer's personality and artistic vision, I think it's best to go to the most direct source: a composer's own letters.
In the days before telephones and the Internet, people frequently wrote letters. In fact, letter writing even became something of an art form, and some composers' letters (like Felix Mendelssohn's) are highly literary. Other composers, like Mozart, had no clue that anyone would ever be interested in reading in their personal correspondence, so they wrote with little or no "filters." That might make Mozart's letters less literary, but they also make them much more salacious. Letters were often intensely personal, so much so that Brahms actually destroyed many of his letters (and demanded that Clara Schumann do the same), deeming them far too personal or scandalous to ever see the light of day. Yet it's the very intimate nature of these letters that give such enormous insight into a composer's inner life, and what makes them such a fascinating read.
Mozart's letters, for example, reveal much about his life and character. Some of his early letters are notorious for their dirty jokes and toilet humour, showing that even great geniuses sometimes enjoy a good laugh. Yet his letters to his wife, Constanza, show a very tender and affectionate love between them. He makes sexual innuendos, apologizes to her for their arguments, and expresses his longing for her whenever they're parted. His letters to his father, who could be a difficult and stubborn man, often show maturity, restraint, and an enormous amount of compassion and kindness. After his mother's death in Paris, Mozart is tasked with informing his family of the tragedy. Though deeply distressed, Mozart handles the situation with supreme delicacy, taking care to send a good friend to give his father support when he learns the terrible news. Mozart's letters give the reader a vivid impression of his character with their personal details and expressive style of the writing itself.
Of course, composers write about their impression of the music of their day, how best it should be performed, and how they compose or get their ideas. Mozart gives his opinion of composers, singers, and the other musicians he works with, expressing his admiration for composers like Johann Christian Bach and Joseph Haydn and singers like Aloysia Weber. He wrote to his father that in opera, the poetry (or libretto) must always serve the music, not the other way around, in striking contrast to the later philosophy of Richard Wagner. In a letter to a Baron who asked him about his creative process, Mozart described how his ideas came to him when he was "alone and in good spirits," he developed them in his mind until they were fully formed, and then wrote them down from memory. Because almost the entire process took place in his mind, Mozart's scores show very few corrections or edits.
Composers' letters offer great insight into their lives, their music, and their character. This can make them very enjoyable to read; they're full of unexpected jokes, intimate feelings, and often brilliant commentary on their society and time-period. I'd highly recommend them to any musician or music lover.