There are many different kinds of intelligence. Some people are book-smart, others are street-smart, and some have an innate intelligence that helps them get through life without relying on either books or the lessons learned on the street by those who've attended the University of Life.
Malcolm X was always a highly intelligent man. Out on the streets, he was, in his own words, "the most articulate hustler of all." In other words, we might say that Malcolm was street-smart, an important characteristic to have if you're going to make a living by committing crime as Malcolm did when he was younger.
But when it came to writing English, such as in letters, for example, Malcolm found himself unable to express himself. This was because he was functionally illiterate, which embarrassed him greatly. Malcolm may have been street-smart, but he certainly wasn't book-smart, and this was something he sought to remedy when he was in Charlestown prison.
While he was serving time, Malcolm discovered the joys of reading. By learning how to read, Malcolm acquired an extensive vocabulary that increased his confidence as well as his stock of knowledge. Reading also enabled him to write letters to Elijah Muhammad in which he could properly express himself and convey his thoughts.