The Letter appears in the second ending of Persuasion. In the first ending, Austen has Captain Wentworth confront Anne about her rumored marriage to Will Elliot. When Anne hotly denies the rumor, she and the captain are able to reach an understanding.
Austen's fantastically intelligent revision produced the current ending: the letter which Captain Wentworth writes after he hears Anne declare that women stay true to their first loves. He is stuck in a parlor, surrounded by people, and responds with the means he has at hand.
It is the perfect pay-off for the novel and for these particular characters (the 1995 movie combines the two endings quite effectively).
Letter-writing was an omnipresent activity in Austen's world. In many ways, it was more like "tweeting" than even modern-day emailing. Consider Pamela in Richardson's novel, feverishly writing her parents every detail of her life. Consider Darcy's letter to Elizabeth or Jane Bennet's continual letters to her absent sister. Consider the romantic poets who were constantly exchanging letters, many of which were written on the backs of poems or in the margins of prior letters (paper was a precious commodity). For that matter, consider Jane Austen's letters to her own sister!
For Austen as for Shakespeare, letters are the ultimate truth-tellers. In Much Ado About Nothing, Shakespeare uses his characters to argue that what one sees or hears cannot be trusted, only what one writes. When Benedict and Beatrice exchange poems at the end of the play--after another prideful argument--Benedict declares, "Here's our own hands against our hearts!"
In Persuadable, Penelope Clay naturally never reads Captain Wentworth's letter, but she guesses at its content:
The next day [the Elliots and Penelope Clay] were all busy getting ready for the party. At least Elizabeth kept herself busy making minor adjustments to the arrangements made by the servants. She walked about the drawing room twitching tablecloths here and there, straightening plates and utensils (there would be light refreshments).
Anne escaped; Sir Walter went off to show himself in the Pump Room. Penelope was left to pace behind Elizabeth, complimenting every minor adjustment: “You are such an observant hostess, Miss Elliot. You have such a flair for perfection.”
Leaving the drawing room to fetch a different vase for a floral arrangement, Penelope encountered Anne coming in from the outside. Anne looked flushed and distracted, her eyes straying to a letter in her hand; she started at seeing Penelope.
“Oh,” she said.
Penelope paused, clasping the vase. Anne had given her a loss of composure last night; this was Penelope’s chance to win some back.
“I was with the Musgroves,” Anne said and blushed.
Not just the Musgroves, Penelope surmised.
“Captain Wentworth has confirmed his attendance tonight,” Anne stammered.
Anne turned away abruptly and shrugged off her cloak. As she lay the cloak across her arm, Penelope noted that she didn’t loosen her grip on the letter.
Penelope returned to the drawing room. So—Captain Wentworth had finally vocalized or written his intentions. This time, Penelope guessed, Anne wouldn’t push him over.
But then surely Anne had always known how she felt; she’d only lacked conviction. Penelope had plenty of conviction. She just wished she knew what to do with it.