We need to clearly understand that the ancient idea of a “book” is quite different from how moderns conceive it. The Hebrew term sefer, and its Greek equivalent biblion, are generally translated into English as “book,” but this is inaccurate. Sefer/biblion literally means “scroll,” and refers to a long narrow rolled piece of parchment (animal skin) or papyrus. Our English word “Bible” derives from the Greek plural ta biblia (“the books”), and would be better translated as “the scrolls.” (When Qur’ān 3:110 calls Jews and Christians the ahl al-kitāb—“the people of the Book”—it means the people of the Bible, that is those who accept the authenticity of biblical scripture.) The Hebrew Bible was originally a collection of numerous scrolls, sometimes rolled on a stick, often stored on shelves, and transported in cases or boxes. The development of modern books, in the sense of a pile of cut sheets of parchment/papyrus/paper written on both sides and bound on one side, did not develop until the first century AD, by which time all of the texts of the Bible had already been written.
In this regard it is important to note that an ancient scroll did not necessarily consist solely of unified composition by a single author. It was often the case that a composition might be longer than the maximum usable physical length of the average scroll, and might therefore require multiple scrolls to contain a single literary work. Hence, for example, Homer’s Iliad was divided into 24 biblia/scrolls/books. Likewise, Plato’s Republic consisted of ten biblia. This organizational division into scroll-length “books” continued even after the development of the codex allowed many separate scrolls to be bound together in a single book in the modern sense.
In a way, the traditionalist view is that the Pentateuch (literally, in Greek, the “five [scroll]-cases”) is one “book” whose length required five separate scrolls to copy. First and Second Samuel and First and Second Kings were also originally a single “book” which required four scrolls. Likewise First and Second Chronicles was a single “book” consisting of two scrolls. On the other hand, we also often find that multiple short compositions by separate authors might be combined together on a single scroll. This is the case with the twelve “Minor Prophets” whose works were generally found together on a single scroll, as is the case in the earliest surviving biblical manuscript of those texts, the second century BC scroll 4QXII from Qumran. What all this means is that the Bible is not a book in the modern sense. It is rather a composition of many different texts of different lengths, from different authors, times and places, which were somewhat arbitrarily split or combined together according to the spatial limitations of more or less standardized lengths of the scrolls upon which they were written. The ancient decisions regarding the physical division or combination of texts into scrolls/books often was only marginally related to questions of authorship.