Without a more specific book location and chapter reference, I can only hazard a best guess as to what exactly Isabel saw and heard in the library. This is because Isabel goes into the library multiple times throughout the book. Sometimes she is sneaking in to gain access to Robinson Crusoe, and other times Isabel is in the library at the direction of the Locktons.
The library is the place where Master Lockton plans and prepares for all of his political intrigues. It's where he hosts other powerful Loyalists to debate how best to defeat and wreak havoc upon the Patriots. Madam Lockton is not allowed in the room during these meetings, but that doesn't mean she doesn't attempt to spy on the meetings by sending Isabel into the room with refreshments for the men. More than likely, this question is asking about what Isabel saw and heard in chapter 14. Lockton, the Mayor, and several other important people are all in the library, and Isabel is sent in. It's amazing how the men completely ignore her presence, and she overhears how their original plan to bribe Patriots didn't work out so well. The men decide that more direct and drastic actions are necessary, and Isabel sees and hears the men begin to plot the assassination of George Washington.