There are two new features in Version 9, one big and one small. The major new feature is Document Drafts — single click draft creation for your entire Document. Drafts work at the Document level, taking a full copy of your working tree and all scenes, and allowing you to refer back to it at at any time without having to use backups or project copies.
Full details in the Knowledge Base here.
The second smaller new feature is Scene Properties — a dialog that shows meta data about each scene and allows you to set options for a scene such as excluding it from exports, word counts, etc.
A detailed list of changes is on the Change Log page here.
And you can download Version 9 here.