This is a curation of books worth purchasing (or borrowing if possible) about the business of artwork.
This book is written by Gwenn Seemel and is described as a book about how art marketing "isn't just about selling more art."
This book is less on selling and more on creativity, but still a great read for an artist.
A book by Alyson B. Stanfield, and is "The ARtist's No-Excuse Guide to Self Promotion"
A bestselling book by Julia Cameron
This isn't specifically targeted to artists, but Google Adwords is a great way to promote your work.
Written by Napoleon Hill. Another non-artist-specific book, but a great read for anyone starting a business.