On your book's product page at Amazon is a link Tell a Friend. A form will appear on your browser with the boilerplate message:
Hello,
I found this item at Amazon.com and thought you might find it of interest.
Ask recipients to forward the message to their friends. But don't go overboard: This form isn't to be used for bulk "promotional" purposes, according to Amazon. So don't use the Tell a Friend feature for people you don't know or anyone who might complain about the message. You don't want to be accused of sending spam.