A Love Letter is a tome that can be found in Act II of Diablo III in Dahlgur Oasis.
Retrieving it adds to the Judge of Character achievement.
It reads as follows:
From the pages of Solan, exalted poet of Kehjistan: My love rises out of the oasis like a willow. Let me nourish her like a pool of cool water. Let me wind myself up her curves and hollows like fingers in ivy, and climb her shaded branches to taste of her fruit.