JAX-RPC supports async messaging and its covered in my book. If you want to learn more about asyn architectures in general, I would recommend the new book from Martin Fowler's series,
Enterprise Integration Patterns : Designing, Building, and Deploying Messaging Solutions. It does a great job of explaining proper asyn messaging architectures - how to use it in real world applications.
[ Tom - changed link to use UBB code ]
[ January 28, 2004: Message edited by: Thomas Paul ]