Notebook pages (also called Game Bible pages) are collectibles that were introduced in Version 1 of Prison Architect. They are scanned pages from the developer's notebook (Game Bible) used to create the game and provide an insight into how the game works and was designed.
To collect pages keep your eyes open for scraps of paper on the floor of your prison during the game and click on those scraps to unlock another page. Previously collected pages can be viewed under 'Game Bible' in the game menu under 'Extras'.