Here are few to read:
- Understanding Microsoft Integration Technologies – A Guide to Choosing a Solution
- Writing Effective BizTalk Server Adapters
- BizTalk Server 2004: Monitoring and Troubleshooting
- BizTalk Server 2004: A Review of the Business Activity Monitoring Features (BAM)
- How to Trigger a BizTalk Orchestration from SAP
In general, I’d recommend to go trough all the technical articles of BizTalk Server.