In addition to free download of the Underworld Kingdom Volume One, now you may purchase a paperback print via Lulu.com. It's price is rather reasonable (at least in my opinion) :-)
Underworld Kingdom Volume One: Explorers of the Unknown - buy a Lulu Print!
EDIT: I don't know when I'll manage to add more of my stuff to my RPGNow webstore. They have ridiculous requirements of PDF preparations which require the newest editions of Acrobat. My old 7.0 is unable to prepare a file properly so I'm sorry - right now I can't add more of my stuff (such as Towers of Krshal or Terminal Space) to RPGNow.