Question

which book do you want后面的do you want还可以用什么？

Answer 1

which book do you want.
后面的do you want还可以用下面句子代替：
1). Which book do you prefer?
2). Which book do you like to have?

Answer 2

Which book would you like?
Which... would you like?
用来询问对方需要什么。是一个常用用法。听上去比Which... do you want? 还亲切。