SOPP504: The hardest part I ever studied was the harmony I never understood

This question was sent by Vitold, and he writes: “Good day! This is Vitold from the organist school. Can you send or recommend literature to better understand organ harmony? Because I write those notes and I can't understand where and how those intervals are calculated. The hardest part I ever studied was the harmony I never understood. I played mostly from hearing and finally I would like to learn. Sincerely Vitold”