选A,正如楼上所说：many修饰可数名词,much 修饰不可数名词.
为什么不能用much修饰more：
这里的book是 一个名词,当more后面跟的是多音节的形容词时,表示比较级,意思是更……,而much此时就变成了形容一种程度,这里有几个例句：
I have many bikes.
I have many more bikes than you.（可数时）
I have much milk.
I have much more milk than you.(不可数时）
She is more beautiful than you.（意思是：她比你更漂亮）
She is much more beautiful than you.（意思是：她比你漂亮的多）