A third letter to Claudia is about Zoroastrianism and the battle between honesty and lies in that belief system. The Persians believed in the value and power of this life as opposed to other religions like Christianity which preach acceptance of this miserable world in hopes of a better life after death. Dick wants to bring back this affirmative view of life.
I expect he’s not done writing to her, since the glossary also notes Dick referred to his early Exegesis as mostly letters to Claudia.