Repair of a book would reinforce and strengthen the structure so that it can continue to be functional. This may require paper repair, resewing, regluing the spine, reinforcing the covers and re-attaching them with strong hinges and joint material.
Antiquarian and rare books may require restoration with conservation considerations. The repairs are made preserving as much of the original historic structure and materials as possible. Care is taken to ensure that materials are conservation quality.
If your book's cover cannot be repaired you may consider rebinding it. Cover materials may not be exactly the same but there are many choices in cloth, papers, and leathers.
You could also have your favorite book rebound with beautiful, artistic binding. A binding can be an artistic expression of the story that you love and can aesthetically enhance the reading of the book or your library.