In parallel with learning SAP it is critical to update and enhance your business knowledge.
In the area of Purchasing and Supply Chain Management, there are some excellent books available. One that I have been recommending is 'The Supply Management Handbook' by Cavinato and Kauffman. This book has been a staple reference for purchasing and supply professionals around the world.
The latest edition (7th Ed, 2006) is a great update on an already fantastic book. The 6th edition (1999) was called 'The Purchasing Handbook'.
Just to make it clear, the book is not about SAP. It is a great reference to setup and manage Purchasing and Supply Chain Processes. If you can affort the book --- get it!