The term 'HEAVY BOOK' can be broken down into two crossword solutions: 'TOME' and 'MARS'. A 'TOME' is a large and heavy book, usually one that is scholarly in nature. The word 'TOME' is derived from the Latin word 'tomus', which means 'to cut'. This refers to the way books were once made by cutting pages from a single sheet of paper. 'MARS' is a reference to the Roman god of war, who was often depicted carrying a heavy shield. This is a metaphor for the weight of a book, as it can be quite cumbersome to carry around.