Sweet and Sour Apples, and Gratitude

Here is one of my favorite passages from The Conference of the Birds by Farid ud-Din Attar, translated into English by Afkham Darbandi and Dick Davis.  Apples seem to figure prominently in Persian literature and even film, often as metaphors; does anyone know why? And does anyone know where I can get Conference of the Birds in Farsi?

 
A good kind-hearted monarch one day gave
A rosy apple to his favourite slave,
Who seemed to eat the fruit with such delight
The laughing king said: “Here, give me a bite!”
The slave returned him half, but when the king
Bit into it it seemed a paltry thing,
Unripe and tart.  Frowning he said: “And how
Is what appeared so sweet so bitter now?”
The slave replied: “My lord, you’ve given me
Such proofs of constant generosity,
I could not find it in my grateful heart
To grumble just because one apple’s tart -
I must accept whatever you bestow;
No harm can come to me from you, I know.”
If you meet tribulations here be sure
That wealth will come from all you must endure;
The paths of God are intricate and strange -
What can you do? Accept what will not change!