You can switch word counting on and off for individual book elements so that you only count words that are really part of your book and not a part of the supporting elements, such the foreword, table of contents or index.
Word counts operate on both the free and paid versions of the plugin, but in the paid version they are also used to report on your progress against target.
Switch Word Counts On/Off for a Book Element
Open the book you want and select an element such as a Chapter section. Scroll to the end of the page and find the Element Options section: