why's that? my pet already have 31 intelligent, and that means she has read 31 books already, I should've get my achievement by now :/

You need to click on the drop down and view the "Books Read" page in order for it to register.

Same with foods, if you're going for the Gourmand achievements.

You can also gain Intelligence without reading books. The mysitc spindles game can provide intelligence and some stat boosters provide intelligence too.

I'm baffled :o, so that's how it works, anywho, I got my achievements, thanks a lot for the information! that's new for me, thanks a lot!