For books where I already know most of the story (especially the ending) I read the intro before. If I don't major events in the story then I read it afterwards. I find that most of the time it is worth to read it though.

Around 800 books (mostly mine but some are my husbands). Non-fiction takes up around a third of it, mainly historical accounts with a focus on the 19th to early 20th century. For fiction it's mostly classics with a focus on european literature.

Yes, most of the time 2 or 3. One fiction and one non-fiction usually.