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When the Persian king Darius invaded Scythia, the Scythians sent him a bird, a mouse, a frog and five arrows instead of a written message. Darius read it as surrender of land and water. The actual meaning was a threat: unless the Persians could fly like birds, burrow like mice or leap into marshes like frogs, Scythian arrows would kill them, showing how object-messages leave the meaning to the receiver's guess.
Picture-writing becomes true writing when a drawing stops standing for the thing it shows and starts standing for the sound of its name, so it can spell unrelated words that sound the same. Once signs carry sounds, names, abstractions and grammar become writable. Repeated simplification of those sound-signs eventually produces a small alphabet that almost anyone can learn.
According to the ancient account, Egypt cut off papyrus exports to stop the library at Pergamon rivaling Alexandria's. Pergamon responded by perfecting a writing surface made from treated animal skins, which took its name as parchment. A trade blockade on one medium produced its replacement, and the new medium could be written on both sides and folded into pages.
The shape of writing and books follows from what they are made of and what they are written with. Stylus-pressed clay produced wedge-shaped marks and papyrus had to be rolled into scrolls. Tough parchment made the bound codex practical, and cheap paper made mass printing economically possible. Each new material solved a limitation of the previous one and changed who could own and read books.