Art is a unique way of expressing one's viewpoint on the world. What makes a work of art art is its link and meaning to its creator. Musicians, visual artists, filmmakers, and other artists cannot be separated from their works because art is so personal.

Art may be a source of money as well as a method of personal expression. By contributing to an artist's work, you are also contribuing to the artist. It isn't until the artist becomes problematic that the connection between art and the artist becomes a problem.

Reading Rowling's books promotes her career and ability to create more content, therefore it's impossible to separate the art from the artist. You can't buy a book from JK Rowling and disregard the fact that she is an outspoken transphobe since your purchase promotes her. It's exactly like boycotting Amazon for how they mistreat their workers. It's all business in the end. You are, in fact, supporting her when you buy something from her. It's that simple. Other than that, art is the greatest expression of an artist.

It is not to say you can't read Harry Potter again. Separating the art and the artist is in the end a personal decision—just like it becomes a personal decision when you choose to support exploitative companies, but to imply that art and its artist can be separated is kind of a long shot. However, it's also to keep in mind how you supporting the said artist is affecting her victims. JK Rowling continues to be transphobic and is getting away with it because people have the notion that she should profit despite her hate speeches. Personal decisions don't always end up being only 'personal' after all. You're promoting transphobia under the guise of "personal decision."