J.K. Rowling published the first Harry Potter novel under the initials J.K.
Rowling was born Joanne Rowling and did not originally have a middle name. Her publisher asked for two initials because research suggested that some boys might be reluctant to read a book written by a woman. “K” was supplied in reference to Kathleen, the name of Rowling's paternal grandmother, creating the professional name J.K. Rowling.
The initials became one of the most recognizable author names in modern publishing. Rowling later used the name Robert Galbraith for her crime novels, allowing her to publish in a different genre without the immediate expectations attached to Harry Potter. The initials should not be expanded as “Joanne Kathleen” as though Kathleen had always been part of her legal birth name.