Am I correct in thinking this is an insult of sorts, or a 'polite' way of telling someone you think they're talking BS? Or do you say it when you genuinely think someone should write an actual book?
So
YABU - it's meant sincerely, recipient should write a book.
YANBU - it's a way of saying you think they're talking a load of rubbish.
Sorry for hijacking AIBU for the voting...