I've just bought vs. I just bought vs. I bought [duplicate]

1.I've just bought a book on English grammar, and it looks pretty good.

This sentence used a present perfect with "just" which indicates buying a book occurred very recently. It can even mean you bought a book a few minutes ago.

2.I just bought a book on English grammar, and it looks pretty good.

This sentence is very close to No.1 in that you used "just" for the past tense.

The purpose of Both No. 1 and No. 2 is (1) to indicate the timing of buyig the book and (2) the fact that he/she has it now.

3.I bought a book on English grammar, and it looks pretty good.

Reading this sentence, you cannot tell whether the book was bought recently or not. It happened in the past. No. 3 is different from No. 1 and No.2 in that sense.