I suggest starting with Complete Method for Autoharp and Chromaharp by Meg Peterson. It includes a recording. It is available from Autoharp Quarterly.
Another one frequently promoted, satisfaction "guaranteed" if downloaded as a trial version, is a private publication by Cathy Britell. See Cathy Britell Select Educational Materials, then It's an Autoharp . Also consider the download option linked at the bottom of the page.