I once proposed doing a book about this, but Paramount asked me to refrain. At the time, they thought it best to keep the matter mysterious, which was a perfectly legitimate call, especially since TNG was still producing new episodes.
Now that TNG is off the air, they might feel differently.
I'd still agree with them. Any attempt of explaining the backstory will ruin it.
And then you'd have to connect her being in the Nexus, her ability to sense timeline changes and her being a match for Q, and Q's comment "Guinan, ah that's what you call yourself now" when she was clearly called Guinan even 400 years earlier and was living on Earth.
You'd turn her into a Q, or into another super being, or into a special El-Aurian, or whatever, and find some contrived reason why she always stays with Picard. In any way, it'll ruin it.