According to Charles Earle Funk’s book Thereby Hangs a Tale: Stories of Curious Word Origins, the Old English word for the beech tree is boc, and it was this tree’s inner bark that was written upon by scribes. Eventually, their writings came to be called book.
Not the most interesting origin story in the world, and certainly not the only place books came from, but it seemed like a good place to start and I really like the book Thereby Hangs a Tale. If you are a word lover, this book will fascinate you.