What is the difference between “Dua buah buku” and “Dua buku”?
“Dua buah buku” is not a phrase that is used to say that you have two books.
(well, at least, I wouldn't use it like this)
I think that the confusion maybe caused by the word "sebuah".
This word "sebuah" is used as a classifier/counter.
However, in this case a classifier/counter is not needed.
"Dua" (two) is used to indicate the quantity.