Runes of the Quest by J.T. Marie
As a child, Morlea's handmaid wove wondrous tales to amuse her. Among these stories was the legend of Talitha, a princess like Morlea herself, trapped in a tower whose walls wept at her unjust imprisonment. An evil wizard hid her away until a prince could save her, then struck the land and its people barren to prevent any attempt at rescue.
Morlea believes the story nothing more than a fantasy until, years later, she discovers a possible location for the fabled tower. Dissatisfied with palace life and looking for adventure, she decides to chase down Talitha on her own.
A princess needs to be rescued, of course. And if there aren't any able-bodied princes up to the task, why can't another princess be the savior for once?