Finance: Accounting, Economics, Finance. There is huge overlap within these areas, particularly at the "lay"level which you need. You're NOT looking to become a finance "pro." There is a link below which ties the three together via a PowerPoint presentation and related Excel file. If you find the suggested texts vexing, you might try breezing through the PPT to gain perspective.
Excel: Every semester on the first night of class, I challenge the dozens of students holding graduate degrees in engineering (or other hard-sciences) to self-identify as Excel "Expert" users by show of hands. Every semester, 3/4 or more of the hands shoot up; fifteen minutes later, a similar request always garners: zero. Everybody has Excel; damn near no one — engineers included — ever bothers to learn to use it. You're not learning (necessarily) to become an Excel guru, however, you'll find that you know far more than the vast majority of business folk you encounter. It will become your secret weapon, as you'll learn.
Quant: Yes, groan, you'll have to make sure that you've a workably sufficient grounding in mathematics. While you'll find that many of the concepts help you to successfully wrestle with business problems, that's NOT the main thrust of your quant undertaking. Rather, you'll find that "consulting" life will go FAR smoother if you're able to frame answers mathematically, most particularly, graphically. Non-quants, in particular, become docile when they feel that they're in "good hands." The "good news," is that the (slim) one-text version should only take a weekend's reading; the multi-text version should only add two or three weekends.
Business: Whatever the sector, ALL businesses share (intellectually) common components. In addition to those of finance noted above, marketing and operations are particularly significant. Whatever the business, you have to produce/"produce" your product and you must market (subsumes, "sell") your product. I do not address, nor, in all candor put much store in, the management discipline, per se. OB/HR (as it's collectively known in the academe) is and has been an utter waste of time for students. No one (period) has figured out how to make a "leader." If you're bright enough to lead, you'll figure out what and what not to do. Most failed leaders I've encountered were merely dummies pushed beyond their modest limits. Harsh, perhaps; nonetheless, true.
Programming: I've only tangentially addressed the matter of Programming. To borrow a classic German, "saw," the first step in preparing Hassenpfeffer is catching the rabbit. Here, the objective is to get to the point that you'll have the knowledge to program something saleable, the skill to market same, and, most importantly, the managerial wherewithal to render the enterprise a success. We leave until later the specifics of converting your "saleable" idea into compiled software ready for distribution. Coding, by the way, is the least of making a success of software. It is a classic instance of a necessary but insufficient condition.