What raw materials are used mainly depends on the publishing house. Now many ancient books are still made of lightweight paper. I even bought a copy from Zhonghua Book Company. This paper is white and thin, almost transparent, and you can see the words on the next page. But it is also Zhonghua Book Company, and I have also bought some ancient books with thicker and stronger paper.
But to say that the best paper for ancient books is probably Guangling Bookstore. Many of them are made of rice paper, which is not as high as usual.