Jolene's Letter is an optional item Link can acquire in Phantom Hourglass. Link will first need to encounter Jolene after completing the Temple of Courage. Link will also need to unite Joanne with the Old Wayfarer, over at Bannan Island. After these requirements are done, the Postman will deliver Link Jolene's Letter.
The letter is deliver in error, as it was intended for Jolene's sister, Joanne. The Postman will then task you with delivering the letter to Joanne at Bannan Island. Successfully doing so will cause Joanne to give Link a Wisdom Gem. Joanne will also send a letter to Link with a Ship Part included.