Jesus
Why didn't Jesus write a book himself?
Jesus lived in a culture where respected teachers, called rabbis, mostly taught through speech, memorized by their students and carefully passed on—not primarily through writing.
His followers eventually did write, but what got written centered on his life, actions, death, and resurrection more than a rulebook of ideas. John's Gospel opens by describing Jesus himself as "the Word"—suggesting that, for Christians, the core message was a person to be known, not only information to be read.
There's also a practical point: a book can't heal someone, share a meal with outcasts, or die on a cross. Whatever the deeper theological reasons, the shape of Jesus' ministry—teaching, relationships, action—doesn't map neatly onto a single written treatise.
Christians hold different views on how much theological weight to put on this question, and it's fair to hold it loosely. What's not in real dispute is that his earliest followers felt a responsibility to write his story down for people who wouldn't get to meet him directly.
Bible quotations in these answers are generally taken from the Christian Standard Bible®, Copyright © 2017 by Holman Bible Publishers. Used by permission. Christian Standard Bible® and CSB® are federally registered trademarks of Holman Bible Publishers. Some biblical wording may be paraphrased or follow other translations.