How does Mr. Lemoncello leave after he asks Charles what lifeline he wishes to use in Chapter 49?

After asking Charles what lifeline he wishes to use, Mr. Lemoncello heads for one of the bookcases on the far side of the rotunda. He tilts back the head on a bus and presses a red button in the middle of what would have been the man's neck. A door-sized section of the bookcase swings open. Mr. Lemoncello steps through and the bookcase swings shut.