Lord Acton: Many essays and articles, beginning about 1860, mostly collected in his History of Freedom and Other Essays, 1906, and Historical Essays and Studies, 1907.
Acton: Lectures on Modern History. 1906. (I use the 1912 edition; the lectures were delivered in 1899-1901).
Acton: Letters to Mary Gladstone, ed. H. Paul, 1904.
Jacob Burckhart: Die Cultur der Renaissance in Italien. 1860.
(English translation by S. G. C. Middlemore, 1878). Twentieth ed. by
L. Geiger, 1919.
W. Stubbs: Lectures on European History. 1904. (Delivered 1860-70).
François Laurent: Études sur l'histoire de l'humanité. 18 vols. Vol. viii: La Réforme. (No date, circa 1862). Vol. xvii: La Religion de l'avenir. 1870. Vol. xviii: Philosophie de l'histoire. 1870. (pp. 340 ff).
John William Draper: History of the Intellectual Development of
Europe. 1863.
Draper: History of the Conflict of Science and Religion. 1874.
W. E. H. Lecky: History of the Rise and Influence of the Spirit of
Rationalism in Europe. 1865.
K. P. W Maurenbrecher: Karl V und die deutschen Protestanten. 1865.