If you consistently want to provide an index for all new folders of a specific class a better approach would be to indicate the index applicability in the folder class before you and your fellow authors start adding the new folders. The next steps demonstrate how to enable the index applicability for all new books that are added to a project folder.
Opening the object classes from the menu bar
Select the 'Window > Object classes' item on the Workbench main menu bar to open the 'Object classes' window.
The 'Object classes' window
Click on 'Organize folder classes'.
Selecting the 'Project' class
Select the 'Project' class and click on 'Show details'.
Setting the 'Index applicable' option in the class
Select the 'Book' folder class in the 'Added classes' on the left window. Next, select the 'Index - Applicable' option and eventually the 'Include topic descriptions in the index on the right window. Click 'Submit changes' on the left window and close the windows.
If you now add a new 'Book' object under a 'Project' folder then the book's 'Index applicable' (and eventually the 'Include topic descriptions in the index') option is automatically switched on.
By default new objects inherit the index applicability from their parent folder. So, by switching on the index applicability in the 'Book' class, also the chapters and subchapters will have the index option switched on.