Paul Auster successfully creates the Kafka world of paranoia where everyone is in on the new deal except you. The book recounts a day in the life of the incarcerated main character, called Mr. Blank because he can't remember his name or much about himself. He looks at pictures of people from his past that he cannot quite remember and is cared for and visited by these same people. He has a strong sense of his own guilt which these characters affirm. He reads a manuscript of a story that ends abruptly and is asked by the doctor to finish the story as part of his treatment. He begins telling the doctor how the story goes, and after the doctor leaves before he can finish the story, Mr. Blank continues the story aloud to himself, as he will have forgotten everything by the next day's session. In the story-telling, he talks about creating the plot -- story flaws, retracting one approach for another, reminiscent of the abandoned plotlines in Oracle Night. John Rause, a character who is an author in Oracle Night turns up here. Perhaps other Auster characters turn up, but I didn't know them.
So perhaps the travels in the scriptorium refer to Auster rummaging around in the plot-room of his brain.