I don't have many details other than Gabriel took Sam as a child from John and Dean. Maybe to spare him the pain in his life?? He took him, erased his memory (maybe Dean and John's too) and raised him as his own. When he saw Sam's love of books he decided to name him 'Book'. Through some series of events Sam and Dean end up together. Sam may know who Dean is but I'm pretty sure Dean does not know Sam. The fic wasn't complete at the time I read it. I am hoping now maybe it is.

Thanks!