That way you'll make the thread easier to read for everyone. It's a breach of copyright to copy entire books into email without permission. Anything posted to a public newsgroup forfeits copyright unless it's stated in the article that copyright is retained. That means that you can usually forward net articles to your pals. You're expected to attribute such forwarded articles to the originator.
Private email is exactly that.
Unless the person has stated that you may forward email from them to someone else, then you should ask their permission before doing so. Remember if you quote someone, quote them correctly, quote them in context, and attach their name to the quote. Most people have screens that can accomodate 80 character lines. If lines go over that sometimes they wrap onto the next line and sometimes they don't. If you have lines more than 80 characters long then some folks will literally only get half the message.