In preparation for a TV special on Abraham, Sarah and Hagar, I've been reading Philo on Abraham and Sarah and Hagar in On Mating with the Preliminary Studies. Philo has some alternative interpretations of unresolved tensions in the biblical narratives:
When, therefore, you hear that Hagar was afflicted by Sarah, you must not suppose that any of those things befell her, which arise from rivalry and quarrels among women; for the question is not here about woman, but about minds; the one being practised in the branches of elementary instruction, and the other being devoted to the labours of virtue.
Philo's allegorical endeavors are a fascinating counterpart to the allegory of Sarah and Hagar in Galatians 4. On the history of the interpretation of Hagar, John Thompson, Reading the Bible With the Dead (Eerdmans 2007) has some interesting things to say. I will post news of when this program will air later this fall.