Core marketing, accounting, finance, and microeconomics. In more or less that order. Others include behavioral economics/decision theory--or at least read Ariely or Thaler's book. Other possibilities include organizational behavior, negotiation, pricing, business strategy. One general comment is that, if you're just looking at a few courses, you'll probably get more out of in-person courses that have case studies and/or group discussions, role playing, etc. and just read a book to cover more lecture-oriented material (like microeconomics usually is).