Emerson wrote Nature, the little book which is the base text of American Transcendentalism, after over ten years of extensive reading. Kenneth Cameron spent much of his scholarly life collecting and publishing these intellectual sources of Emerson's thought, most notably in Young Emerson's Transcendental Vision, through his Transcendental Books press in Hartford, Connecticut. His massive collections, listed in his Bibliography on Transcendentalism or the American Renaissance [formerly Transcendentalism and American Renaissance Bibliography], bring together manuscripts and reprints by Transcendentalists, newspaper articles, lists of library reading and key passages from those readings, and much else. Many of these materials are useful and illuminating, though usually fragmented and presented with little context, critique, or ordering. One might think of them as a massive, unlinked hypertext, useful primarily when linked to the context of Transcendentalism as a way of understanding the works and the people.
The Transcendentalists, Emerson in particular, read widely and appropriated ideas freely and eclectically from their reading. Emerson and Thoreau both kept notebooks in which they recorded choice passages, and those are increasingly available to scholars for study. When they drew on these ideas in their works, sometimes they explored them in some depth, as Emerson did in his essays on Montaigne and Swedenborg; more often we find the ideas greatly modified and pulled together into the fabric of their own ideas, with little attribution. Margaret Fuller was particularly interested in German literature, translating and writing Dial essays on Goethe and Bettina von Arnim [see Arnheim's book on her correspondence with Goethe]. The study of their reading and how they used it has attracted many scholars, primarily in academic source studies. Joel Myerson's The Transcendentalists: a Review of Research and Criticism is the best source for finding many of those.
Emerson was profoundly influenced by European philosophical and religious thought, as well as literature, while he thought through his ideas about nature before 1836, as his journal and notebooks show. Samuel Taylor Coleridge, as introduced to the Transcendentalists by James Marsh, was especially influential. (See Yoder thesis)
Emanuel Swedenborg. Emerson was introduced to Swedenborg through the agency of Sampson Reed's "Observations on the Growth of the Mind." Cameron has listed ideas and values that Emerson found in Swedenborg's ideas (at least, as conveyed by Reed). Emerson would later write an essay on Swedenborg [Representative Men. 1850.]
Platonism and Neo-Platonism. Emerson acknowledged his great debt to the thought of Plato in two essays in Representative Men, "Plato; or the Philosopher" and "Plato: New Readings." Nature is grounded in Platonism, especially Neo-Platonism, as embodied in the writing of the Cambridge Neo-Platonists such as Ralph Cudworth.
The transcendentalists were all dedicated, life-long readers. Emerson, Thoreau, and Alcott were especially attracted to Oriental philosophy and religion, reading in translations available to them and copying favorite passages (from Confucius, Laws of Menu, Hinduism, Buddhism, and more) into their personal notebooks and from there into the Dial, their prose and poetry [see Emerson's Hamatreya, Saadi, and Brahma, for example.] Tracking these influences can be difficult, as David Ch'en's essay on Thoreau and Taoism shows. See also East Meets West: Oriental Seeds in Occidental Soil. by Swami B. G. Narasingha and Satyaraja dasa (Steven Rosen).
There were also profound influences from American thought and literature. Foremost may have been the eloquent sermons of Reverend William Ellery Channing of Boston, which anticipated much of Transcendentalism, in particular its philosophy and opposition to slavery.
Bryan Hileman, VCU