At that point in the story Holden is in a worse state than ever and clearly not far off hitting rock bottom in his life. Just when it seemed he had found an adult figure that he could relate to and was starting to listen to some advice he feels terribly let down. Whether or not Mr Antolini was actually behaving like a sexual predator is not made clear in the story, but from Holden's point of view he was doing "something perverty" (p. 200) and therefore the deep confusion he feels about human relationships (particularly with adults) continues.
Whilst waiting for the elevator Mr Antoloni tells Holden again that he is a "very, very strange boy" (p. 200), which obviously is not very helpful for Holden's state of mind. Holden then goes through that moment of uncomfortable silence that we all experience when waiting for (or riding in) an elevator with others. Of course in Holden's case the moment is far more embarrassing because of what had just occurred in Mr Antoloni's living room. In a desperate effort to say something he comes up with "I'm gonna start reading some good books. I really am" (p.200). This is presumably in response to Mr Antoloni's earlier monologue about Holden needing to involve himself with academic education as he is clearly "in love with knowledge." (p.196)
What Holden says to Mr Antoloni is really quite hollow, but he says it because he is just desperate to say something and then take leave from a highly uncomfortable situation.