Here are some great Microsoft SAP resources

SAP and SAP BW data in PowerPivot
using-sql-server-powerpivot-for-excel-to-analyze-sap-data.aspx.

SAP HANA data in PowerPivot
http://www.sdn.sap.com/irj/scn/weblogs?blog=/pub/wlg/26747.

Microsoft Connector

http://msdn.microsoft.com/en-us/library/dd299430.aspx

BizTalk Adapter

http://msdn.microsoft.com/en-us/library/cc185427.aspx

XML/A Provider with .NET

http://msdn.microsoft.com/en-us/library/cc974473.aspx

SAP Excel ODBO connector, see:

https://www.sdn.sap.com/irj/scn/go/portal/prtroot/docs/library/uuid/1031a475-a633-2a10-f4b4-ccfe615c0517

Multiple SAP BI interoperability papers

http://www.microsoft.com/isv/sap/technology/interop/bi.aspx

SSIS and SAP BI White Paper

http://msdn.microsoft.com/en-us/library/dd299430.aspx

SSRS and SAP NetWeaver BI Technical Article

http://msdn.microsoft.com/en-us/library/cc974473.aspx

Microsoft .NET and SAP - Interoperability

http://www.microsoft.com/learning/en/us/Books/12513.aspx

SAP Business Information Warehouse:

http://help.sap.com/saphelp_nw04/helpdata/en/e3/e60138fede083de10000009b38f8cf/frameset.htm  

ODBO flattening algorithm:

http://msdn2.microsoft.com/en-us/library/ms709733.aspx