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.