Aside from the last, each room in the Epilogue has at least one object that you can stand behind. If you have the red book on that screen open and the text of it is displayed in the air, standing behind these objects in the proper location will cause a sound to play and the text to change, revealing more information. If you open a green book, the red book will close and the text will disappear, so you will need to use time manipulation to see all 4 of these secret writings.