Obvious question for your follow-up question: what makes you think books are commonplace in D&D worlds?
Absolutely right. Fact is most in this world are illiterate and it's really only the scholarly few who can truly read. Scrolls, books, etc are not cheap nor are they incredibly common place. The facilities to create them are also quite few and it's why basic parchment is considered of value. The only reason why we might feel books are commonplace is because the heroes we play are legends with gifts and skills above the norm, and we surround ourselves with like-minded individuals of accomplished adventurers.