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.
That probably wouldn't work if the browser crashes, but I'm not sure. You could also send a periodic request from every page (like once a minute or something) and if that "keepalive" request doesn't come in the session is invalidated.