How did Douglass' narrative effect the public's perception of slavery and its practices?

This depends on which generation of public you mean. Are you referring to the public at the time or the modern day public?

Both.

I think Douglass's narrative at the time he wrote it was monumental. I don't think it had changes ingrained opinions but it did expose the cruelty of slavery from the point of view of an articulate Negro man. This, in itself, planted the seeds of slow change. Today, his narrative is shocking but is both a lesson and a reminder of human nature's darker side.