The reader does not know exactly what is in Anna's letter to Sarah. This is because only Sarah's reply to Anna is shown in the book. Sarah's letter to Anna does, however, give clues about the first letter. Sarah responds to several of Anna's questions.
In her letter, Anna asks Sarah if she can braid hair. Anna's mother had died many years before, and since then no one has been around to braid her hair. Sarah responds that she is able to braid hair. Anna also asks Sarah if she can cook stew and bake bread. Sarah replies, saying that she can indeed do both. After writing this, Sarah adds that she prefers painting and building bookshelves to baking and cooking.
Anna may have asked Sarah what her favorite colors are. In Sarah's letter to Anna, she states that her "favorite colors are the colors of the sea, blue and gray and green, depending on the weather" (Sarah, Plain and Tall, Chapter 2).