Who is Libanius and Priscus from Julian and what is their importance?

Libanius and Priscus are apparently old friends, and the book is a compilation of Julian's words interrupted by observations of the two men, Libanius and Priscus. Both are elderly at the time of the writing and both knew Julian. Priscus was mentor, teacher and fellow soldier for Julian before he became Augustus. Libanius has decided that he would like to publish a book of Julian's memoirs and requests a copy of Julian's biography that has somehow come into Priscus' possession. Priscus agrees to have the manuscript copied for LIbanius, but requests that his name not be associated with the finished work. He does, however, offer his own point of view throughout the book, sometimes clarifying Julian's words and other times correcting Julian's memory.