1. Apple D, Watson P (eds), Grzybowski A, Ludwik Zamenhof: a major contributor to world culture, on the 150th anniversary of his birth, Survey of Ophthalmology, 2010 55(2).
2. Forster PG. The esperanto movement (reprint). Berlin: de Gruyter Mouton; 2012.
3. Penarredonda JL. The invented language that found a second life online. BBC Future. 2018; https://www.bbc.com/future/article/20180110-the-invented-language-that-found-a-second-life-online 5 January 2022.
4. Tonkin H, editor. Zamenhof: the life. Mondial, Bergisch-Gladbach: Works and Ideas of the Author of Esperanto; 2010.
5. Zamenhof LL. An attempt towards an international language international language Henry (reprint). London: Forgotten Books; 2012.