Engel, Gerhard. “Paul Frölich: Im Radikalen Lager. Politische Autobiographie 1890-1921 [review in German]”. The International Newsletter of Communist Studies (July 24, 2017): 97-99. Accessed May 19, 2019. https://incs.ub.rub.de/index.php/INCS/article/view/706.