attached to the secrecy as to the high
Hal likes to get high in secret, but a bigger secret is that he’s as attached to the secrecy as he is to getting high.
Context
The single sentence that reroutes the whole habit: the concealment is not overhead on the pleasure, it is the pleasure, or a second pleasure equal to it. What looks like a boy hiding a vice is a boy who needs the hiding — the No-One-Must-Know is load-bearing, and naming it "a bigger secret" makes secrecy the actual object of the attachment.