They were making something that would be recognizable as a book (or booklet) at least as early as late republic, which ties in to my point that the format has been refined a long time.
I think you mean the codex? It was like a book, except the pages were bound on alernating sides. Not the best design, they improved on it in the middle ages. The codex achieved parity with the scroll in 300 AD, so that the modern book format coming just 200 years later means it wasn't around for that long.
