I know that he had boxes and boxes - a complete apartment full - of notes he had made about things like life on Krypton, the complete Kryptonian language (which he had fully developed - a vocabulary, grammar, alphabet, etc), and more.
I think those boxes possibly went to Rich Morrisey and/or Al Turniansky when he passed on, I'm not sure. Nor do I know where they are today.
But I do know that they are invaluable, and are the real treasures of his estate.