SAP BusinessObjects BI 4.0 The Complete Reference 3/E Contains 5 chapters on SAP Dashboards plus a lot of other information on the BI4 platform.
Information Dashboard Design: The Effective Visual Communication of Data Improve your dashboard design and usability, regardless of which dashboard tools you use.
SAP BusinessObjects Dashboards 4.1 Cookbook Helpful for users of Xcelsius and Dashboards 4.1. Love the "cookbook" format.
Getting Started with SAP BusinessObjects Design Studio SAP Design Studio for creating the next generation of SAP dashboards.
Performance Dashboards: Measuring, Monitoring, and Managing Your Business The single best book on business intelligence for both business and IT professionals.