To respond briefly to the question, the kanji “本” began to be used as the meaning of “book” later than it began to be used as a counter. Today, we will detail the origins of this kanji and how it started to take on the meaning of a book.
(One of the major ones out of the many theories.)
Thank you very much, fawa2idさん and Mateuszさん, for these great questions on Twitter!
First of all, let's get to know this kanji character
1. the origin of, the base of, main (+noun)
2. correct, formal (+noun)
3. true, real (+noun)
4. this (+noun)
5. my, our (+noun)
6. counter for counting sticks, string, or other long, thin objects
Origin of the kanji character “本”
“本” was initially used as a counter to count trees. When counting trees, they counted the number of the roots (shown above the ground) of the trees and drew a line on the tree to mark that they were already counted.
The kanji “本” was produced by drawing a single line at the tree’s root in the pictograph of the kanji “木” (“tree”) to represent this counting work.
That’s why we still use “本” as a counter for counting some thin-and-long shaped things such as trees, sticks, and strings.
So, how come it started to mean “books”?
The root of a tree can also be considered as “the base of a tree,” right? So over time, people started to use this kanji also to mean “the base or foundation of things.”
Later, around the 6th century, Japan began to adopt ideas (religions) such as Buddhism and Confucianism from the Asian continent, and it became necessary to read many books written in Chinese characters (written in Chinese) to learn those teachings.
Since the content of the books was the base of religious teachings, the written teachings themselves came to be called “本.” This is the origin of the term “本” for books today.
When you studied Japanese, you first learned “book” as the meaning of the kanji for “本,” didn’t you? So it is often thought that “book” is the original meaning, but in fact, it has a longer history of being used as a counter.
What's the counter for 本 (books)?
We use the “冊” counter for books.
Before the paper was invented in China, people wrote characters on narrow slips of wood or bamboo called “木簡” (もっかん) to record their writings. Since they were too narrow to write many characters, they were tied together with strings and used like paper.
The pictograph “冊” was formed from how they connected with this string. And this is how it became a counter to count the number of books in Japan.
By the way, this kanji is also the source of the Korean and Vietnamese languages, which use it to mean “book” in Korean (책) and Vietnamese (sách).
Special thanks to fawa2idさん and Mateuszさん!