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By far the best book on SAP Workflow is the third edition of Practical Workflow for SAP, by Jocelyn Dart, Mike Pokraka and many others.
If you are at all serious about learning SAP Workflow, get this book first!

The SAP Community Network Workflow Forum is a useful site for searching for answers to SAP Workflow questions and problems, or posting your own (registration is required) :

SCN Workflow Forum

The SAP WUG (Workflow User Group) mailing list contains more than a decade of valuable information about SAP Workflow. Always worth searching when you encounter a problem you haven’t seen before. You can also become a member and receive daily digests in your inbox. To search the mailing list archives (e.g. for RFC error) :

Search WUG

A custom Google search for SAP Workflow issues, searches in both WUG and SCN, as well as SAP Help and various other related sites.

SAP Workflow search