Found this post on the NSS Facebook page:
"There was an announcement that was distributed by The Speleogenesis Scientific Network yesterday, you can find it here: http://www.speleogenesis.info/ftp/SGN_D ... CH2013.pdf
As you can see from the advertisement, you must purchase the books as a package in order to receive the 50% discount. But, not to worry Amy-- when you receive your May NSS News, you will find an advertisement specifically for NSS members. We will also announce it here on Facebook on May 1. Rest assured, there are plenty of copies of these books to go around!"