[Air-L] Searching about teaching in virtual worlds

Hi everybody! I´m doing a little research about virtual worlds as a tool for teaching... Can you recommend me some bibliography about it? I´ll preciate If you know some free pdf documents available in internet; by reason of prices, some books in english are very difficult to afford in my country. I have another question: do you know some free alternative to videogame "Minecraft"? Think in using it for educational purposes. Thanks, Alejandro Tortolini Journalist - Teacher - Researcher Buenos Aires, Argentina