Knowledge and Choice Solution

Shalom Rabbi, regarding the last episode in the series of God and the World on knowledge and choice, we were taught back in high school that just because God knows does not mean we have no choice, meaning He knew what we chose and it is not dependent on the choice, just as if you had a letter that you open at the end of the day and see that He knew what you did all day even though you had a completely free choice. Regarding how it is possible to know something that did not happen, I can barely make the excuse that perhaps He can 'calculate' or know what a person's heart is inclined to and therefore He knows what he will choose anyway.
My question is why his knowledge would necessarily prevent free choice, it's like the story the rabbi brought about the book that knows everything, the person who read it had complete free choice that ultimately led him to his death, and it's not necessarily related to opening the book or what the book knew or didn't know. I didn't completely understand why this contradicts
thanks