Mu You had seen this item on the forum before, it was specifically used to increase the level of magic.
The level of a spell can be upgraded from LV1 to LV2 simply by increasing the number of times it's cast, but to upgrade beyond LV2, these pages from the corresponding Element Comprehension Book were necessary. They came in three levels: primary, intermediate and advanced. To upgrade from LV2 to LV3, primary comprehension pages were needed, from LV3 to LV4, intermediate comprehension pages were needed, and to advance from LV4 to LV5, advanced comprehension pages were needed.
Depending on the level of the magic, the number of comprehension pages required for upgrading also differed. For instance, primary level magic needed just one page to upgrade, intermediate level magic required five, and advanced level magic needed ten!
As for levels above LV5, no external items were of any use. One could only upgrade by gaining personal insights through constant use.