I learned a lot about how this business runs through reading the forums. LOL
One of my favorite reads (though I hate how everything for him is a sales pitch) is Rich Dad, Poor Dad. His quadrant idea was genius. He also said before you take on liabilities, go out and get assets! Basic business 101.
I also have studied a lot on Real Estate through understanding what they're going through and how the operate in Rental vs. Flip options in both Residential and Commercial (which are both different beasts)
... and a TON OF MARKETING AND SALES BOOKS/COURSES. The amount of money I've dumped into that is unreal haha... Though I will admit, I learned a lot more from my Marketing/Sales mentor than those books can ever teach you.