The first thing any designer should read before going out into the real world is Adrian Shaughnessy’s How to Be a Graphic Designer without Losing Your Soul. Most of it applies to all designers, but a good chunk of it goes into the world of freelancing and running a studio. If nothing else, it should give you a pretty good idea of what you want to pursue.
We’re constantly shaping our creative process, but we’d highly recommend both Designing Brand Identity by Alina Wheeler and The Design Method by Eric Karjaluoto. The latter is fairly dense but an invaluable tool (we wish we would’ve found it years before we did). The former takes a bit more of a marketing-ey approach than we do, but it’s good to have multiple perspectives on an approach. Don’t be afraid to take only the things that work for you and make the process your own; we’re constantly experimenting ourselves.
When it comes to the business sides of things, we learned a lot from the Graphic Artist’s Guild Handbook of Pricing and Cameron Foote’s The Creative Business Guide to Running a Graphic Design Business. The latter has some practices that might be considered a bit outdated, but 90% of it still applies. Aside from book resources, we can’t understate how important is was to learn directly from professionals that were willing to mentor us.