In Plato's time, people learned information primarily through verbal discussion, then shared it with their friends, family, and community to pass it on through the generations. Oral tradition was extremely important to their culture at the time because it was accessible to everyone, and required practice and recitation to ensure no details were left out. Not many people could read back then, and those who could might not be able to read at the level to understand more complex topics.

It was also thought that writing down information would give people an excuse to never properly learn or discuss it in the first place, therefore depriving society from the uninterrupted chain of gaining and passing on knowledge. Anyone who could read could find the information, but would they actually understand it? This is why public forums were so important back then; you needed a community to share and discuss knowledge so everyone could understand and learn from it.

Also, it was Socrates who hated writing and refused to document any of his philosophy teachings. The only reason we know he exists today is because Plato took extensive notes on the teachings of Socrates. If anyone was pro-book, it would've been Plato.