Red Gem Book : MM20
It seems you pressed the correct button.
Starting from the circle and moving clockwise, the shapes increase by the number of vertices.
Triangle, square, an unknown shape, and then octagon increase in the number of vertices in order.
Therefore, the correct answer is the button for the shape that has a number of vertices between four and eight.
Soon, a part of the wall beneath the buttons opens, and the book treasure set with a red gemstone is revealed.
You take the treasure from the small space inside the opened section of the wall and secure it.
After obtaining the treasure, you leave the area.
You make your way out.
You successfully secure the treasure.
Congratulations!