Hello,
I can recommend:
www.workflowbook.com
Workflow is usually used for approvals, e.g. for an employee requesting Leave.
Generally, it is used for situations where work passes from one person to each other (e.g. in a multi-level approval process) and it facilitates the passing of that work.
But, it can do so much more - you could even use it as a sort of ABAP++ :
What can't it do? Good question. I think it's more of matter of what it shouldn't be used for. Anything that can be done in a simpler way, I suppose. It shouldn't be used to replace complicated transactions.
regards
Rick Bakker / hanabi technology