There are over 4 million words of US tax law. It's a lot to dissect and hardly any of it is relevant to the average small business.
You can't go wrong with a personal finance 101 book. You can skip around and still get a decent primer on the US tax system.
Not Your Dad's CPA has a bunch of videos targeted towards small business, lots of information on reselling in particular. Sole proprietorship vs LLC vs S-Corp, small business 101 type stuff.
For stock trading, good intro post about long term vs short term gains. This is a deep field especially when you consider wash sales, covered call tax implications, etc. See Investopedia for specific questions.
All about payroll taxes- in case you have employees.
Lastly, I found that reading about how wealthy individuals and large corporations abused the US tax system helped me better understand it. Some of these tricks are applicable to small businesses, but only some. A Fine Mess: A Global Quest for a Simpler, Fairer, and More Efficient Tax System