The below 3 books are the best books for SAP BI.
1) A Practical guide to SAP Netweaver Business Warehouse(BW) 7.0 By Bharat Patel, Amol Palekar and Shreekant Shiralkar
This books will give you give complete overview of SAP BI 7.0. It covers complete reporting(Query Designer, Bex Analyzer and WAD). Everything is explained clearly.
2) Data Warehousing With Sap Bw7: Bi In Sap Netweaver 2004s : Architecture, Concepts, And Implementation By Christian Mehrwald and Sabine Morlock
This book doesn't cover reporting part, but very good book for complete modelling and about the issues we get realtime.
3) SAP Business Information Warehouse Reporting By Peter Jones
This 3 books are enough for SAP BI.