The crossword solutions for the term TEXTBOOK ADDENDUM are INDEX, which is a five-letter word. An INDEX is an alphabetical list of topics found in a book or other publication, usually located at the end of the text. It is used to help readers find information quickly and easily. An addendum is an additional section added to the end of a book or other publication, often containing new or updated information. Therefore, a textbook addendum would be an additional section added to the end of a textbook, containing new or updated information.