Do you know of any works/writings/audios/etc by him that are rare that people can read? What did you learn when you met him? thanks

ב"ה Shalom I only have some of Rav Aryeh Kapalan's zt"l, sefarim which I cherish. I don't have anything of his that was not published. Artscroll came out already in 1985 with "The Aryeh Kaplan reader" which is a collection of essays of his. In this book there is also a short biography of him. This is in addition to his many books and booklets which he himself had written and published. In the short encounter I had with him, I remember seeing a very modest amicable person full of patience for all those waiting on line to speak to him or ask questions. I just wanted to ask him about one of the references in one of his books. I don't remember which one. I only remember his answer ,which was "מושבע ועומד מהר סיני"= which freely translated means that a Jew was already sworn in at Mt. Sinai to keep the mitzvot, which is a phrase which appears many times in the Talmud. (יומא עג:, נדרים ח. And more) May his memory be for a blessing.