The term BOOK BACK is composed of two words, BOOK and BACK. The first word, BOOK, is a noun that refers to a physical object consisting of a number of pages bound together. The second word, BACK, is an adverb that indicates the rear or reverse side of something. In the context of a crossword puzzle, the solution for the term BOOK BACK is SPINE, which is a noun that refers to the thin strip of material that runs along the BACK of a BOOK and holds the pages together.