Ashurbanipal holding a stone book?

I am wondering what Ashurbanipal is holding. it looks like a book. is it some kind of historical artifact related to him? and he surely has a kingly voice.

It's a tablet, which were the main form of writing before books were invented.

Yes, it's a large clay tablet which has cured. Generally, most examples we find of tablets are hand-sized and depict trade figures or other documents intended as contracts. I imagine it wouldn't be very epic seeing him holding a small tablet in his hand, so the artist used some license.