Sombart, Werner: Socialism and the Social Movement, 1909. Widely read, both in the original and in the English translation. Contains an interesting critique of Marxianism.
Spencer, Herbert: The Coming Slavery, 1884. A reprint from The Contemporary Review.
Stoddard, Jane: The New Socialism, 1909. A convenient compilation.
Tugan-Baranovsky, M.I.: Modern Socialism, 1910. A systematic and scholarly résumé of the doctrines of Socialism.
Warschauer, O.: Zur Entwickelungsgeschichte des Sozialismus, 1909.
Wells, H.G.: New Worlds for Old, 1909. One of the most popular expositions of Socialism.
MARX AND ENGELS
Aveling, E.B.: The Student's Marx. A handy compilation. 1902.
Boehm-Bawerk: Karl Marx and the Close of His System. An English translation was made in 1898.
Engels, Friedrich: Die Entwickelung des Socialismus von der Utopie zur Wissenschaft, 1891.