For the Halloween 2013 event, TF2 added spellbook and spellbook page items. The pages can be added to the spellbook, and the book shows what pages have been added to it. However, I have not noticed any difference in using the spellbook after adding pages. So, what changes when I add pages?
Adding pages to the fancy spell-book do not affect the game-play/use of the item.
The only difference it makes is it increases the counters on the items loadout stats.
Source: tf2 wiki