http://www.sap-press.com/products/SAP-BusinessObjects-Web-Intelligence.html

This is probably the first Web Intelligence specific book on the market; it will definitely be the most comprehensive.   It should be available sometime in April 2010.  As Webi continues to evolve and add features, its great to finally have a book devoted just to Web Intelligence reporting.  I look forward to reading it this spring and picking up some new Webi tricks.