People Without a Written Language

Until almost 1920, the Navajos as well as other Native American tribes did not have a written language.
Between 1910 land 1920, an effort was made to learn the language and put the sounds in phonic language. Up until that time, much sign language was used or oral language, which took interpreters.
Most of the old biographies were pass on though the years by family historians, and not well known by many of the tribal members.
Stepping Stones to Navajo History has some great biographies.