現在完成式問題

I haven't bought this book yet.
這句是你想講的意思-我還未買(要加yet暗示我想買但未買，否則只是表示現實上我還未有買)

I don't buy this book.[不可能/不可以加yet.]
這句是斬釘截鐵告訴對方「我是不會買這書的」
意思跟你想講的完全不同。因為是用present tense(否定詞)表達是真理(即是現在如此永遠如此)的立場，所以是「我永不會買這書的意思」。這寫法可使用任何動詞都表示這種意義：I don't go there = 我是(永遠都)不會去那裏的。

I haven't bought this book yet. Yet is usually used in the "Present perfect tense." This means 我還未買這本書. This sentence indicates that you have not taken an action.

You do not put "yet" when using the present tense.
I don't buy this book. That means this is not happening. Or you can say:
I don't want to buy this book which indicates you have no desire of doing so.

沒有分別

只是I haven't bought this book比較正式

I don't buy this book yet文法沒有錯, 亦可以當作書面語, 但更常出現於對話上

當然有分別,前面那句就對,但後面那句是說「我還不買這本書」,這就錯啦!