Anthony Grimani states you have two types of diffusors, 2D and 3D. He stated to use the 2D diffusors in the front half of the room and 3D diffusors in the back half. The 2D diffusors have to be deep enough to impact certain frequencies so a book case wouldn’t cut it. You can check out Auralex Metrofusors for 2D. 3D diffusors by Auralex include the Geofusor, the the Tfusor and the Hemisphere. The Tfusor and Geofusor can also be filled with polyfil or rockwool if you also want to give them some absorption capability. I used Auralex Sustain series diffusors in my setup that are made from bamboo but were discontinued. You can check Anthony’s brand, Sonitus: I really like this video: