Final answer:
Ralph Waldo Emerson is the writer and thinker of the 19th century who wrote the book 'Nature', and is recognized as a key figure in American transcendentalism.
Step-by-step explanation:
The individual considered one of 19th century America's greatest writers and thinkers, and who wrote the book 'Nature', is Ralph Waldo Emerson. Emerson was a central figure in the transcendentalist movement and his work 'Nature', published in 1836, is foundational to transcendental philosophy. His essay and subsequent lectures laid the groundwork for a new, distinctly American thought and philosophical strand, emphasizing the importance of the individual, self-reliance, and the spiritual experience of the natural world. Emerson's influence can be seen throughout American literature and thought, and his writing played a role in shaping the nation's cultural identity during that period.