I've only been able to locate the developer guide from 6.7.1 SP1.
I can't find any more recent guides on the customer portal site... Is only the WF user guide available for 7.3.2?
All the documents were rewritten/updated for 7.1.0, and then only those that were updated will be available in the downloads section of later versions. Generally speaking though, the 7.1.0 docs are close enough.