I am going through the Mark Reads and Mark Re-Reads writeups of Harry Potter, and one thing that really stands out to me is how much fun it was reading the Harry Potter books as they came out, with the rest of fandom alongside me.
Because the thing is, fandom is smart. And brimming with theories. I mean seriously! Fandom caught on to Arabella Figg before I did (and wrote long gratuitous fics about her as the new DaDA professor). Fandom caught onto the significance of R.A.B. = Regulus Black = locket from Book 5 when I would have forgotten it.
Nothing in Book 7 surprised me too much because all of the out-there stuff was a theory that had at one time or other already been proposed by somebody in fandom. Seriously. I'd read fics about all the major plot twists. This didn't spoil the book for me or ruin it for me because there were also umpty-million theories and ideas and fics that weren't in the final book, but it was tons of fun.