Reading is one of my top hobbies. Hoarding books is another, but I'll save that story for later. The material I choose is heavily dependent on my mood at the moment, so I tend to have phases when it comes to selecting books to read.
Before I quit my full-time job in June, I went into hyper-research mode about living the freelance life. One of the best resources I've read on the topic was Meg Ilasco and Joy Cho's Creative, Inc. It covers a wide range of subjects like finding motivation and inspiration, working through a creative slump, marketing your self and your works, and dealing with the technical stuff like contracts and pricing. I also have other books that belong to the same series (Art, Inc. by Lisa Congdon and Craft, Inc. by Meg Ilasco), but I feel like this book has the right balance of topics for someone like me who deals with both arts and crafts customers as well as illustration clients.