You might want to check out the cirricula of thw MBA programs of a couple of big business schools (Michigan, Whatron, Kellogg - places like that), and then buy the textbooks that are used in the required courses.
most business schools have required courses along the lines of:
Marketing
Accounting (usually both Cost and Financial)
Finance
Strategy
Organizational Behavior
Operations
Economics
Statistics
A lot of the technical stuf flike Financial Accounting, Finance, Stats, etc - are not really that useful per se, but can be very important in order for people to get jobs. Plus, a huge number of MBAs at the top schools take an ass-load of Finance classes so they can get an I-Bank job in Manhattan and print their own money (and retire at 40).
I'd recommend getting some textbooks (or similar books) on Strategy, Organizational Behavior, Marketing, Operations and Economics. Look around for the previous editions - they'll usually be a lot cheaper, but often have exactly the same content.
Spend some time working your way through those, and you'll already have a realyl good idea of most of the "bones" of what an underlying management education is all about.
(Obviously, that is only scratching the surface, and MBA students go ont to take numerous electives to specialize in their particular majors - but for a general insight into basic business principles, I think you'd be off to a good start).