Graphic Artist's Guild Handbook of Pricing and Ethical Guidelines
This is a very helpful book for doing art within the professional field, especially as a freelancer. Even though it focuses on digital art and graphic art, it includes lots of information on contracts, copyright, your rights as an artist of any medium, and more. Much of the information applies to all mediums.
You do not necessarily need the newest one as they produce a new one every year and it contains mostly the same information. However it is definitely essential for reference and providing you all the information you could ever need with doing art professionally.
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