Keep in Touch: Letters, Notes, and More from The Sisterhood of the Traveling Pants, by Ann Brashares, is a companion piece to the author's book The Sisterhood of the Traveling Pants. The book has no plot of its own, but rather is a collection of notes, letters, and other items that the original book's four major characters have written to each other during the course of their lifelong friendship. Much of what happens in Keep in Touch is vague and somewhat confusing because the author assumes that the reader is familiar with the original book.

The Sisterhood of the Traveling Pants follows four teenage girls - Lena, Carmen, Bridget, and Tibby - who have been close friends all of their lives. During their 16th summer, they are separated for the first time when Lena goes to visit her grandparents in Greece; Bridget goes to soccer camp in California; Carmen goes to visit her father in South Carolina; and Tibby stays at home in Bethesda, Maryland. Shortly before the summer begins, the girls buy a pair of jeans that magically fits all of them perfectly despite the fact that they are very different shapes and sizes. While they are apart, they ship the pants back and forth among them, along with letters and notes keeping each other up to date on what is happening during a very eventful summer that changes each of their lives.

Keep in Touch traces the girls' history back to their childhoods through the inclusion of birthday party invitations and notes that show the reader how their personalies were shaped from an early age. Bridget is very focused on sports; Lena loves art; Tibby is very bright and articulate; and Carmen is a "girly girl" who is deeply affected by her parents' divorce.

Tibby's notes and other writings reveal that she is unhappy about being stuck in Bethesda, working at Wallman's discount store, and babysitting her younger siblings. She meets a younger girl named Bailey, whose presence is annoying at first until they become close friends. As the summer progresses, Bailey becomes very ill, is hospitalized, and apparently dies, although the exact circumstances are unclear in this book.

Carmen's excitement at spending the summer with her father in Charleston, South Carolina turns to dismay when he reveals that he is getting married. Carmen takes an instant dislike to her soon-to-be-stepmother, Lydia, and her children, Krista and Paul, primarily because she resents their intrusion into her relationship with her father. By the end of the book, it is not clear whether or not she resolves her anger toward her father and his new family.

Bridget's summer gets off to a good start, but seems to go awry because of resentment toward her parents, who apparently pay little attention to her. She becomes interested in one of her coaches, Eric, and the book hints at a sexual experience going awry. There are also signs that Bridget is experiencing some serious psychological problems, but the exact nature of these problems is never made clear.

Lena has the best summer experience. She loves Greece and is happy to have so many beautiful places and objects to draw and paint. She also meets a boy named Kostos and falls in love with him. Shortly before returning home, she overcomes her fear of revealing her feelings to him.

The book ends with a section advising readers, who are presumably teenage girls, on various ways to keep in touch with their own friends through written communications such as e-mail, instant messaging, and handwritten letters.