请根据下面短文回答下列问题: Henry Richardson is working in an important library. His job is to give new life to old books. Many of these books are very old or are the one and only copy in the world. Richardson has saved(挽救) the lives of many books. His first job is to wash the pages. He takes off the old cover, and places each page in water. Then he carefully puts each one between sheets (纸张) of paper to dry. Some of the pages may be very thin or have missing parts. Richardson mends these while they are still wet. He places each one on a heavy sheet(块) of glass. Then he carefully puts small pieces of paper on the places which need mending. These pieces of paper are very thin. They look like part of the old paper. The washed pages are now white and clean. The pages are put together again in the old cover. At times, only the front and back parts of the old cover can be used. Sometimes a whole new cover must be made. Now the stronger, cleaner, and "newer" book is returned to the library, once again ready for use. Richardson mends old books ______.