Final answer:
Captain Beatty visits Montag when he starts to question his job and motives, revealing that he knows about Montag's secret library of books.
Step-by-step explanation:
The character who shows up at Montag's house almost exactly when he begins to question his job and motives is Captain Beatty. Beatty seems to have an uncanny knowledge about Montag's internal conflict and reveals that he is aware of Montag's secret collection of books. Although no one has informed Beatty directly, he suspects Montag's actions due to his experience with other firemen who have undergone similar doubts and transgressions. Captain Beatty's unexpected visit and his knowledge about Montag's secret library indicate that he is keenly observant and possibly monitoring the firemen under his command for any signs of deviation from their book-burning duties.