I have read a large part of the book; the rest ____ (be) more difficult.
book 是单数,一本书去掉了一大部分,剩下的肯定小于“一”,所以也是单数,所以谓语动词用单数,填 is。
I have read a large part of the book; the rest is more difficult.
这本书的大部分我都读过了,其余的部分比较难。