Original: Georgiana May, a friend of Stowe's, wrote a letter to the author, saying, "I was up last night long after one o'clock, reading and finishing Uncle Tom's Cabin.
Simplified: For instance, Georgiana May, a friend of Stowe's, wrote a letter to the writer. In the letter, she said that "I was up (awake) last night long after one o'clock, reading and finishing Uncle Tom's Cabin.