If you're learning English, keep in mind that columnists and other journalists will often use very unconventional flourishes in their writing, sometimes with no precedent set by other writers. English is a language with a lot of options to express one's self in the same or similar ways, and most writers take advantage of that. This one is not that unconventional in particular, but it's important to know in general.
Getting to the point, the columnist here is expressing reverence for the author. His statement is literal in a sense -- he is saying that the letter "K" belongs to the author Kafka -- but we, as readers, know that is not possible. It is a sort of hyperbole or big exaggeration. The letter "K" is very closely associated with the author, and often English speakers say that something "belongs" to someone when they are very well known for using it. The author expresses his admiration by essentially stating that Kafka is influential enough to own the letter "K" in the English language.