I have mentored freelancers and agile workers in my field for a while.
One good resource that often gets mentioned is
Freelancers Union. While it is US-based a lot of the resources apply to anyone anywhere, ideas on working with clients, tools, software, etc. If you are US based there is good info on paying taxes (always drama) and you can even sign up for health insurance through it. The Resources and Blog tabs are especially useful.