The Swan Maiden by Howard Pyle illustrated by Robert Sauber

Someone is stealing pears off of the king's prized pear tree.  He promises his sons half of his kingdom before he dies and the rest of it after he dies if they can catch the thief.  Sons #1 and #2 fail in the mission but the third son cleverly stays awake to see the thief, a beautiful maiden who turns into a swan.  She is under a curse by a three eye witch.  In order to marry her, he must completes impossible tasks.  Using her magical powers, the Swan Maiden, helps the third son achieve each task to result in a happy ending.
-Borrowed from the library.