What was in the letter Don Fernando found? Chapter 27

Don Fernando found a letter that Lucinda had sent to Cardenio. The letter expressed Lucinda's love with such clarity and energy that Don Fernando found himself in love with Lucinda, and he resolved to have her. Don Fernando sends Cardenio back to the Duke's palace and, in Cardenio's absence, befriends Lucinda's parents ‹ultimately forcing her hand in marriage.