This is due to how Microsoft Word works. With certain types of forms it will not let you switch to another Microsoft Word document while a form is open in another.

If the form will be working with the document after finishing it needs to keep the document it is working on active. Switching between open Microsoft Word documents will cause Microsoft Word to get confused over what the 'active' document is and it may try to work with the other document instead after completing the form which will result in errors. To prevent this Microsoft Word blocks you from switching to another Microsoft Word document while a form is open in another.

To get around this you will need to copy the text from the other Microsoft Word document before launching the Book Design Wizard form or tool. If you have a lot of different text you need to copy you can also first copy and paste the text you need into Notepad or other non Microsoft Word document before launching the Book Design Wizard form and use that to copy from instead of a Word document.