A book is not a magic trick. It isn't ruined if I know what's about to happen, or how the author did it. Even great mystery novels can be reread a dozen times, because its merit isn't in that singular moment of revelation, but in the gentle weaving of character, place, and plot that creates a world for our minds. I may be soured for a bit... my dad unwittingly did the "Did you get to the part where Ned Stark..." thing when I was reading Game of Thrones. I was so annoyed. But... I still had a complete emotional reaction INCLUDING surprise and disbelief when I did get to that part.
Because a novel is more than the sum of its parts.
when my Dad spoiled that... when my friend thoughtlessly spoiled Harry Potter by asking "Did Dumbledore die yet?", I made a face, I got mad, then I shook it off and went back to reading.