Garrett's new task is to reach inside the central tower, out of the four in the ruins. This is because that is where the book, which you are looking for, has been hidden. You should find the above screenshot helpful, while navigating - your destination is the tower 3, and after you follow the linear corridors and take a simple jump above the chasm, you should be starting in tower 1.
Ignore the ropes above your character, in tower 1, for a moment, because Garrett will use them later on, on his way out of the ruins. Walk through the smashed stained glass window and after you get outside on the balcony, interact with the turning wheel here. Using the valve will cause the first stairs to change their position and you will be able to take them to get to the external balconies on tower 3.
Ignore the first plank, located on the left, go straight ahead and take a turn to the left only after you reach the stairs to tower 5. After you get inside, jump down to the lower level and collect the Poppy.
Take the bottom exit from tower 5 and find a new turning wheel, which you, of course, need to interact with. Using the wheel will result in the turning of another stairs and displaying a cutscene featuring an unidentifiable creature.
Do not worry about the fact that using the wheel made you unable to use the nearby stairs, because you are doing everything according to the plan. Turn back and enter tower 5 again. Approach the wall and climb up onto the balcony above, which you earlier jumped off. Now leave this tower by taking the stairs to tower 3 again.
Take the plank on the side now, which you have earlier ignored, and which leads up to tower 2. Here, you will find the last wheel. Interact with it and take the new stairs immediately, which appeared to the right of your character, after the element of the structure turned. This means, of course. That you will reach the higher level in tower 3.
Walk over the balcony above and stop in the spot shown in the above screenshot. You need to jump down here onto the stairs that you gave a turn earlier, which will open the way for you to tower 4 (the location in which the creature earlier appeared).
Climb up the stairs and, after you have walked into tower 4, walk through the room with the empty crate. You can now take the last stairs, shown in the above screenshot. Take a run-up and jump through the hole in tower3's wall, which will take you to the location where the book is.