Possible Clues:

• American philosopher, this writer entertained by Elizabeth and Charles?
• Writer takes an age to pen manuscript without ruler
• Thoreau's mentor
• Poet Ralph Waldo --
• Ralph Waldo --, US poet, essayist, and transcendentalist
• Iconic keyman Keith
• Thoreau pal
• Leader of the Transcendentalism movement
• American writer, having put out book, remains active
• American poet and essayist's first sermon in new compilation