let you in on it

Montagの上司BeatyはMontagが密かに本を隠しているのに気付いているようです。
Beaty puffed his pipe. "Every fireman, sooner or later, hits this. They only need understanding, to know how the wheels run. Need to know the history of our profession. They don't feed it to rookies like they used to. Damn shame." Puff. "Only fire chiefs remember it now." Puff. "I'll let you in on it."
最後に出てきた表現 "let you in on" は慣用句の様です。辞書で調べます。
"let someone in on something" は慣用句として次ぎの説明を見つけました。
・Macmillan Dictionary: let someone in on something to tell someone a secret: If you promise not to tell, I'll let you in on a secret.: They were planning something, but they wouldn't let me in on it.
・Cambridge Idioms Dictionary: to tell someone a secret Bill, could you let me in on your techniques for getting along with girls? The plan was so well guarded they only let three people in on it.
こう言う簡単な口語表現も知らないとチンプンカンプンですね。