Learning about plurality

I am not plural, but a friend of mine who I hold very dearly is. They're a DID system with many alters, though I am yet to meet all of them. As much as I try to put in the time to research everything about DID, I feel like there's so much that can only be learned from the people themselves. I really, really want to know more about plurality (specifically DID in this case, but all information is welcome!) to help my current friends and any more that may be to come, so if there's anything anyone is willing to share, feel free to. I want to know everything, the good the bad the ugly the awesome. I'll take anything vague, anything in detail. Even the smallest, most common knowledge thing is appreciated.