But most labeled prequels were written after other works, and depict events earlier (for the main character, and unless time travel was involved, for everyone and the world as well). So do you read it in the order of published, or in the order from the character's POV?
In my experience, prequels are best read in the order published, not the in-universe time-line. (Even The Chronicles of Narnia, where Lewis thought you should start with The Magician's Nephew.) On the whole it's because you then get the writer's trade-offs between mystery and dramatic irony. For the first books, he has set up the whole story with revelations and surprises. He expects the reader to be in ignorance and foreshadows all; he expects the reader to be surprised at his dramatic revelations. All the work is aimed at these ends. Ideally it would work as well for dramatic irony (which it needs some of anyway to survive being read a second time).
Then the prequel rolls about. It's for people who've read the first books. Others, too, but it's for them, as well. There's the dramatic irony of the reader knowing how it turned out -- in the opening situation of the earlier books -- vs the characters' ignorance. There's the allusions and in-jokes that lend significance to passages. Ideally all those passages would serve other purposes so the new reader will appreciate them for those (though he will never get quite the experience that a reader of the earlier books would). Ideally the story would work as drama and suspense as well as for dramatic irony.
Then, how many ideal books have you read?