David Airey has a second book out called Work for Money, Design for Love. I haven't read it and therefore have no idea how good it is. He's pretty successful though, so I suspect it's decent.

I also recommend the Freelance Switch blog, which is about freelancing in general, not just design.

A decent book on self employment or starting a small business is probably the best bet, since the principals of finding a buyer for your services ultimately are the same. Running your own design business is more about the business side than the design side; finding clients, keeping them happy, handling finances and taxation, maintaining a website/blog/social media. You'll be surprised how little design you actually get to do sometimes.