When you use the Print-to-file option to create a book, it will open in the word processor that is associated with RTF (Rich Text Format) files. I use Microsoft Word, so my instructions are specific to Word. Other word processor work similarly.
When the book opens in Word, I hold down the "Crtl" key and press the "End" key and then the "Enter" key while still holding down "Ctrl". (This takes me to the end of the document and adds a page break. i.e. creates a new page.) On the new page I click the "Insert" menu, and select "Indexes and Tables". (Some versions of Word have "References" listed instead, so select References" and then "Indexes and Tables".)
I usually opt for a simple 2-column index, and then save the document as a Word .DOC file.
An index is a handy feature for finding people in the book! It is well worth the extra 5-10 minutes.
Also putting a page break at the start of the book makes it easy to create a Title page, Acknowledgements, etc.