Because some people just don't seem to know the rules.
When Discussing a Book
1. You're telling Johnny about a book that you love. He says he's interested in reading it.
"I liked parts of it a lot, but some of it is a little unrealistic."
-this is the only acceptable way to discuss the merit of a book with someone who hasn't read it.-
"Yeah, I liked it up until the part where Bob died."
"Well, it's good, but the escape from the Castle of Certain Doom is sort of dumb."
"But I do love this part where Bob gets together with Jill!"
2. Johnny asks you what the book is about.
"It's an adventure story about some people who have to survive in a tumultuous kingdom ruled by an evil lord, and how they deal with it."
"It's about this guy named Bob who falls in love with Jill. They get taken to the Castle of Certain Death and have to escape and stuff, but in the end Bob dies."
3. Johnny is reading a book that you love, and you want to know how far into the story he's gotten.
"What's Bob doing?"
"Where are they right now?"
-never ask about romance. there is no subtle way to do this.-
"Has Bob died?"
"Have they escaped the Castle of Certain Death?"
"Have you gotten to the kiss?"
4. You've now determined where Johnny is in the story.
*nod* "Cool. Keep reading."
"Ohhhhhhhhhhhhhhhhhhh. THAT part."
"So you haven't--you don't know about--he hasn't--OH EM JEE."
5. Johnny has now finished the book.
There are no secrets now. You may now yap freely.
Further Tips Rules:
1. There is great value in speaking in page numbers.
2. When discussing the story with someone who hasn't finished the book, always speak as though you're at the same place as they are. For instance, if you know that Bob is going to stab Jill in the back in the next fifty pages, talk like you still think he's great.
3. If you can't restrain yourself from spoiling a story, do not say anything. Do not suggest books. Do not read in the first place. Leave the country. And above all, do not ever discuss literature with ME.