Haeckel, Ernst
"Die Welträtsel: Gemeinverständliche Studien über Monistische Philosophie" by Ernst Haeckel is a philosophical work published in 1899. This influential book presents Haeckel's monistic worldview as a solution to the world's great riddles, grounding all development in Darwinian evolutionary theory. The biologist-philosopher examines humanity's place in nature, the soul's material basis, and the unity of the cosmos, while sharply criticizing the Catholic Church and proposing monism as a bridge between religion and natural science. (This is an automatically generated summary.)
These books are free to read. Each is in the public domain in the United States; check your own country's rules before you copy or share one.