if i say i don't think much of this book,this means that-

not think much of somebody/something
means:
to not like someone or something very much
eg:I didn't think much of his new girlfriend.
与之相对：
think highly of somebody/something
also think a lot of somebody/something
means:
to admire or respect someone or something
eg:
Your boss must think highly of you if she gives you so much responsibility.