Okay I'm doing a cake reception for 200. The 3 tiered cake needs to be round cakes 12", 10" 8". That will give 118 servings. The stacked books (2 books) will be another cake. I don't know what size to use for sure for it to look right and to get close to the 82 servings.
Using Earlene's serving size, you could do (2) 10" squares for the books. According to her chart, each 10" square is for 40, so you would be 80.
Rather than sq books use 12x16 or 12x18 - ezch 2" tall will give you a total of 109 servings.
I'm picturing the books stacked so you would slive it 1x2x4 resulting in 109 slices