Robert Schumann
Advice to Young Musicians, translated by Henry Hugo Pierson (1815-1873). Project Gutenberg identifies this edition as public domain in the United States.
Wolfgang Amadeus Mozart
Mozart: The Man and the Artist, as Revealed in His Own Words. This public-domain Project Gutenberg edition contains the excerpts used from Mozart's letters and remarks.
Pyotr Ilyich Tchaikovsky
The Life & Letters of Peter Ilich Tchaikovsky by Modeste Tchaikovsky. The app uses brief excerpts from this public-domain Project Gutenberg edition.
Johann Sebastian Bach
Johann Sebastian Bach: His Life, Art, and Work by Johann Nikolaus Forkel, translated and expanded by Charles Sanford Terry. The app uses brief excerpts from this public-domain Project Gutenberg edition.
Ludwig van Beethoven
Beethoven's Letters, Volume I and Volume II. The app uses brief excerpts from these public-domain Project Gutenberg editions.