The History Compare function lets you view two types of changes - depending on the object.
Properties: Use the Property Sheet view to quickly identify changes to the objects properties (such as the template it is based on, the release state it is in, and so on). The properties sheet displays the settings from the Properties window for the selected object.
The Book object also displays the structure layout of the book as nodes; if you have made several changes to the structure then the last change made to the layout before saving is highlighted. Note, topic content cannot be displayed from the book's property view.
Preview: Use the Preview mode to identify differences to content.