I like the book that is fond of?
I like the book he is fond of.
这里的that可以理解成指示代词而不是充当主语的that?
你的第一个句子有问题:
误:I like the book that is fond of.
正:I like the book he is fond of.
你的第一个句子应在 that is 之间插入必要的主语,如he, she 等。
第二个句子是对的,可视为 he 前面省略了that或which。