Beethoven, Ludwig. Letter, Beethoven to Hans Georg Nägeli, 19 August, 1817. https://brieftext.beethoven.de/henle/letters/b1160.phtml
Beethoven, Ludwig. Letter, Beethoven to Hans Gerog Nägeli, 9 September, 1824. https://brieftext.beethoven.de/henle/letters/b1873.phtml
Budden, Julian M. Knapp, Raymond L. Ludwig van Beethoven. Encyclopædia Britannica. Chicago: Encyclopædia Britannica, Inc, 2020. https://www.britannica.com/biography/Ludwig-van-Beethoven. Also available in print.
Burkholder, J. Peter. A History of Western Music (9th edition). New York: W.W. Norton & Company, Ltd, 2014.
Harding, Henry Alfred. Analysis of form in Beethoven's sonatas, 36-38. Borough Green, Kent: Novello, 1901. https://hdl.handle.net/2027/coo1.ark:/13960/t8ff4bp61. Also available in print.
Marretta-Schär, Luise, and Matthias Thiemel. 2001. "Nägeli, Hans Georg." Grove Music Online. Accessed 1 November 2020. https://www.oxfordmusiconline.com/grovemusic/view/10.1093/gmo/9781561592630.001.0001/omo-9781561592630-e-0000019536. Also available in print.
Pennell, Elizabeth Robins, Pennell, Joseph. Lithography and Lithographers; some chapters in the history of the art, 5-36. New York: The Macmillan Company, 1915. https://archive.org/details/lithographylitho00penn. Also available in print.
Wallace, Lady. Beethoven's Letters 1790-1826, Volume 1 of 2. New York: Boston: Oliver Ditson & Co. Online, 2004. https://www.gutenberg.org/files/13065/13065-h/13065-h.htm#let30. Also available in print.