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The easiest and best option for you, in my opinion, would be the Word Web App or Google Docs, which both offer multi-user editing and version control.

You could try using Google Cloud Connect. I would imagine the commenting would occur within Microsoft Word itself (using the comments feature within Microsoft Word), then once a comment is made the changes are synced with whoever is synced with it. Oh even just use Google Drive or Dropbox to sync and share manually. There is no Google Docs equivalent to ...

There's also Book Writer which let's collaborators/editors view content and add notes and changes and is specifically designed for people writing books (I know, there's a hint in the name). It's free and web-based and from the FastPencil people.