The first thing I will say is this. Two books often get mentioned in this context (including by me). They are 'Music: The Business' by Ann Harrison (UK-centered) and 'All You Need to Know about the Music Business' by Donald Passman (US-centered). I think they are both excellent books, and the sorts of things you should have read. But it needs to be pointed out that they are both very limited in their scope -- they are really written for musicians who are looking to be signed to serious labels, and it tells them about the legal stuff, contracts, percentages, etc. I always thought Passman's book should have been called 'All You Need to Know About the Legal Aspects of the Music Business'. That stuff is all an extremely important part of the business, but only a part. Those books don't tell you much about all the other stuff you need to know. So just be aware of that. There are plenty of other books that talk about the other stuff. (I've read quite a few, but none stood out for me as worthy of mention. I'm going to try The Indie Band Survival Guide than Sthum mentioned).
A really great UK resource for newcomers is bemuso.com -- try bemuso.com/musicdiy/ and bemuso.com/musicbiz/. Again, there's lots of issues which aren't covered here (nothing about getting gigs, getting managers, etc), but it's a good place to start learning.
Here are a few links to SOS articles on starting your own record company and related issues:
A Guide to Contracts for Producers
Recording Contracts Explained
Start Your Own Record Label
Thunder - The Band as Business
Running Your Own Record Label Part 1
Running Your Own Record Label Part 2
Running Your Own Record Label Part 3
Running Your Own Record Label Part 4
Running Your Own Record Label Part 5
Running Your Oen Record Label Part 6
Running Your Own Record Label Part 7
Designer Label - Mike Paradinas & Planet Mu
Nigel Shaw Independent Musician