The Last Little Blue Envelope by Maureen Johnson
Ginny Blackstone thought that the biggest adventure of her life was behind her. She spent last summer traveling around Europe, following the tasks her aunt Peg laid out in a series of letters before she died. When someone stole Ginny's backpack—and the last little blue envelope inside—she resigned herself to never knowing how it was supposed to end.
Months later, a mysterious boy contacts Ginny from London, saying he's found her bag. Finally, Ginny can finish what she started. But instead of ending her journey, the last letter starts a new adventure—one filled with old friends, new loves, and once-in-a-lifetime experiences. Ginny finds she must hold on to her wits . . . and her heart. This time, there are no instructions.