Here are a few links to get you started:
- WebSphere Product Family Overview and Architecture
- Patterns: Implementing an SOA using an Enterprise Service Bus
- XML on z/OS and OS/390: Introduction to a Service-Oriented Architecture
- WebSphere Web Services Information Roadmap
- WebSphere Version 5.1 Application Developer 5.1.1 Web Services Handbook
- Using Web Services for Business Integration
You can download and read the PDFs for all of these.