Mozart in Vienna
All Mozart biographers struggle with the same problem: The later the year, the more meager the documentation.
The composer's early years were chronicled in some detail by his father, Leopold, who hoped to write a biography of his son. Leopold was an acute observer and a prolific correspondent. As a result, our knowledge of son Wolfgang's burgeoning career as a child prodigy is quite complete.
Later, Mozart almost continually exchanged letters with his father. These letters in particular are an invaluable resource to biographers: They reveal the composer's thoughts on music, his personal life (though these comments often must be taken with a grain of salt) and, on occasion, relay details of Viennese social life. Unfortunately, as Mozart came to depend less on his father for support and advice, these letters gradually tapered off before stopping altogether with Leopold's death in 1787.
Mozart wrote few letters after that. Those that he did write were to Constanze when she was away at Baden taking the cure and on those rare occasions when he traveled without her.
Mozart kept no personal diary, though beginning in 1784 he kept a detailed list of his compositions. Thus we have a nearly complete record of the music he composed. But we know very little about what he thought of it -- or of anything else. Mozart's mature career in Vienna, which seemingly should provide the richest material for biographers, instead has lent itself to conjecture and embellishment because we know so little for certain.
Since the beginning of the nineteenth century the resulting lacunae in his biography have been obscured by legends and anecdotes, all stemming from questionable sources -- hearsay or oral accounts that were written down decades after the fact and elude any attempt at historical verification. These embellishments have found their way much too easily into the biographical literature on Mozart and can still be encountered today -- a trusted and cherished ballast that romantically transfigures our picture of Mozart and is certainly useful for marketing purposes. ”— Volkmar Braunbehrens in the introduction to "Mozart in Vienna"