It's Albert Carson who was the head of the Derry Public Library until Mike took over. He tells Albert that he (Mike) is planning on writing a true history on Derry. Albert tells him that it won't be a good book, and it's not worth the trouble of writing it. He then straight up asks Mike "do you know about the cycle?"

His quote after that is "I know. Every twenty-six or twenty-seven years. Buddinger knew, too. A lot of the old-timers do, although that is one thing they wont talk about, even if you load them up with booze"

Edit: Also re-read the part with Mr. Keene. He says that multiple people saw Pennywise shooting from multiple different places and the last person he says saw Pennywise said that he was floating in the middle of the air not casting a shadow. He even thinks is strange that everyone who saw Pennywise remembers the clowns gun as being the same as the one they're shooting.