Microsoft has published the documentation for Microsoft Office Communications Server 2007 R2. It follows the typical deployment process, so it's the first place you should look for instructions on setting one up. Here's the download link.
At the very end of it is a Developer Guide which "provides references to a wide variety of Microsoft Office and Unified Communications SDKs and API sets." You got that right, it's really just a set of links with a few paragraphs explaining the purpose of the link. If that's all you're lookiing for, I'll save you some time by posting them here!
Office Communications Server 2007 R2 Server SDK Unified Communications Managed API 2.0 Core SDK Unified Communications Managed API 2.0 Speech SDK Unified Communications Managed API 2.0 Windows Workflow Activities Office Communicator 2007 Automation API SDK Unified Communications Client 1.0 SDK Unified Communications AJAX 2007 SDK Office Live Meeting Service API Office Live Meeting Service Portal API
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Chris Eargle Telerik Developer Evangelist, C# MVP