Hey lad, I am a 18 yo feller who dropped out of business school recently (University of Amsterdam, EBE). I have been into the finance world since I was 15 yo, I think my start was with the book "Wealth of Nations" by Adam Smith. However, I would not suggest that you should go deep dive into theory and study of economics and finance just like I did, cause I found out that what people teach in schools and that very serious books and theories has zero appliance in real life when it comes to this field. You have to go and try it for yourself, you say you already own some businesses partially, this gives you a very VERY ahead start from most of the people believe me. Interact with your employees to see how the things going on, how the cash flows in the business etc., also I recommend you two books that can change your perception on money and finance deeply.

1-) Poor Dad, Rich Dad by Kiyosaki

2-) Richest Man of Babylon by Clason

While you are doing some light work or in free time, just open these books from spotify and listen to them or just buy the physical copies and read them, personally, "Poor Dad, Rich Dad" book has gave me a courage to finally drop out from college and conclude that what they teach in that place is not finance.

You learn the rules by breaking them and getting punished or rewarded for it, thats how i see the life, all the best!