When writing systems were invented/created in ancient civilizations,

A book is a set of written, printed, illustrated, or blank sheets, made of ink, paper, parchment, or other materials, usually fastened together to hinge at one side. A single sheet within a book is called a leaf, and each side of a leaf is called a page. A set of text-filled or illustrated pages produced in electronic format is known as an electronic book, or e-book.
Books may also refer to works of literature, or a main division of such a work. In library and information science, a book is called a monograph, to distinguish it from serial periodicals such as magazines, journals or newspapers. The body of all written works including books is literature. In novels and sometimes other types of books (for example, biographies), a book may be divided into several large sections, also called books (Book 1, Book 2, Book 3, and so on). An avid reader of books is a bibliophile or colloquially, bookworm.
A shop where books are bought and sold is a bookshop or bookstore. Books can also be borrowed from libraries. Google has estimated that as of 2010, approximately 130,000,000 unique titles had been published.The word comes from Old English "boc" which (itself) comes from the Germanic root "*bok-", cognate to beech.[2] Similarly, in Slavic languages (for example, Russian, Bulgarian, Macedonian) "?????" (bukva—"letter") is cognate with "beech". In Russian and in Serbian and Macedonian, another Slavic languages, the words "???????" (bukvar') and "??????" (bukvar), respectively, refer specifically to a primary school textbook that helps young children master the techniques of reading and writing. It is thus conjectured that the earliest Indo-European writings may have been carved on beech wood.[3] Similarly, the Latin word codex, meaning a book in the modern sense (bound and with separate leaves), originally meant "block of wood".
History of books

When writing systems were invented/created in ancient civilizations, nearly everything that could be written upon—stone, clay, tree bark, metal sheets—was used for writing.The study of such inscriptions forms a major part of history. The study of inscriptions is known as epigraphy. Alphabetic writing emerged in Egypt about 5,000 years ago. The Ancient Egyptians would often write on papyrus, a plant grown along the Nile River. At first the words were not separated from each other (scriptura continua) and there was no punctuation. Texts were written from right to left, left to right, and even so that alternate lines read in opposite directions. The technical term for this type of writing is 'boustrophedon,' which means literally 'ox-turning' for the way a farmer drives an ox to plough his fields.
Tablet
A tablet might be defined as a physically robust writing medium, suitable for casual transport and writing. See also stylus.
Clay tablets were just what they sound like: flattened and mostly dry pieces of clay that could be easily carried, and impressed with a ( possible dampened) stylus. They were used as a writing medium, especially for writing in cuneiform, throughout the Bronze Age and well into the Iron Age.
Wax tablets were wooden planks covered in a thick enough coating of wax to record the impressions of a stylus. They were the normal writing material in schools, in accounting, and for taking notes. They had the advantage of being reusable: the wax could be melted, and reformed into a blank. The custom of binding several wax tablets together (Roman pugillares) is a possible precursor for modern books (i.e. codex).[4] The etymology of the word codex (block of wood) also suggests that it may have developed from wooden wax tablets.