Hi,
I suggest you to consider the latest IWs 8.5 Developer's Guide and API Reference since it is updated regularly to keep you up to date about the new introduced features/functionality in IWs releases.
You have the following docs for your reference:
1) .NET Developer's Guide and API Reference
2) Extension examples
3) Advanced Customization implies usage of Enterprise Service API Reference
With respect to the question itself what do you mean by "complete Interaction Workspace API Reference"? I mean what is your expectation exactly? What criteria you evaluate the completeness by?
Roman T