This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
This will only work on your own pages, but if the user comes from anywhere outside your site, from say www.google.com you won't be able to get a "previous" url, because you could be coming from anywhere, and to me even if you could that would be a security risk. I don't want you to know that I was coming from www.somepornsite.com