This is the first time I read Yusuf Ali's introduction to it and I thought it was quite good:
In subject-matter this Sura is entirely taken up with the story (recapitulated rather than told) of Joseph, the younger (but one) of the twelve sons of the patriarch Jacob. The story is called the most beautiful of stories (xii. 3) for many reasons:(I have yet to read that)
(1) it is the most detailed of any in the Qur'an;
(2) it is full of human vicissitudes, and has therefore deservedly appealed to men and women of all classes;
(3) it paints in vivid colours, with their spiritual implications, the most varied aspects of life - the patriarch's old age and the confidence between him and his little best-beloved son, the elder brothers' jealousy of this little son, their plot and their father's grief , the sale of the father's darling into slavery for a miserable little price, carnal love contrasted with purity and chastity, false charges, prison, the interpretation of dreams, low life and high life, Innocence raised to honor, the sweet "revenge" of Forgiveness, and Benevolence, high matters of state and administration, humility in exaltation, filial love and the beauty of Piety and Truth.
...The Qur'anic story...is a highly spiritual sermon or allegory explaining the seeming contradictions in life, the enduring nature of virtue in a world full of flux and change, and the marvelous working of God's eternal purpose in His plan as unfolded to us on the wide canvas of history. This aspect of the matter has been a favorite with Muslim poets and Sufi exegetists, and is further referred to in Appendix VI (at the end of this Sura), in connection with Jami's great Persian masterpiece, Yusuf-o-Zulaikha.