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

Samples