A 45-year-old librarian has worked at the same library for 20 years. He has never married, has no close friends, and consistently declines invitations to social events from his colleagues. He describes his hobbies as reading and cataloging his stamp collection, both of which he does alone. When interacting with library patrons, he is polite but brief, showing very little emotional expression. He reports feeling content with his solitary lifestyle and has no history of unusual perceptual experiences or paranoid thoughts. Based on this presentation, which of the following is the most likely diagnosis?