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Mira's Letter to Haedrig is a tome in the first act of Diablo III. It is needed to complete the achievement A Quick Study and can be found in the Blacksmith's Belongings in New Tristram, after completing the A Shattered Crown quest.

It reads as follows:

"My dearest Haedrig,
Do not feel despair, my love. You did everything you could. Our time together meant more than words can say, but in the end, fate is a cruel mistress. Your strength is needed to end the horrors that beset this world. My final wish is that you find your path.
Love always,