I think that the poet uses this word to invoke the idea that your mind is a place that is both quiet and in some way holy or above the physical world.
A real cathedral is a place where you go to be quiet and to escape from the reality of the every day world. The services in a cathedral and the meditation you can do take you to a higher plane of existence.
I think that the poet is saying that a similar thing happens when you read. When you read, you can retreat into a cathedral where you are taken away from the physical realities and problems of your everyday life. You are instead transported to a different plane of existence.