gusset

(via Book size - Wikipedia, the free encyclopedia)
"A series of terms is commonly used by libraries and publishers for the general sizes of modern books, ranging from “folio" (the largest), to "quarto" (smaller) and "octavo" (still smaller)." 

(via Book size - Wikipedia, the free encyclopedia)

"A series of terms is commonly used by libraries and publishers for the general sizes of modern books, ranging from “folio" (the largest), to "quarto" (smaller) and "octavo" (still smaller)."