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.
I would suggest searching through IBM Redbooks and IBM developerWorks. I have never worked with anything related to AS/400, but I believe you might need to write some kind of a TCP socket application for calling a proxy server written in Java, which then translates the stuff you write through the socket into web service calls. But, as I said, I've never worked with AS/400.