Added Mage's book.

Customizations.md

@@ -50,6 +50,9 @@ This page is a comprehensive list of changes that Snowscape has made from the va
 | Black | Onyx Enchant | Black Dragonhide |  | Not implemented at the moment (it can be crafted but won't carry anything). |
 
 ### Modified Items
+- The Mage's Book, earned from the Mage Training Arena, has been given the ability to swap between unlocked spellbooks.
+	- To unlock this ability, the spellbook needs to be added to the Mage's book. Take the Mage's book to the altar you pray to a spellbook, and use the book on the altar to add it.
+    - Once unlocked, equip the book and operate it, then select the spellbook you want to switch to. This cannot be done in combat.
 - Crystal bow and crystal shield have been heavily modified. They no longer get weaker as they degrade, and the number of charges has increased from 2,500 to 12,000.
 	- The recharge cost has also changed. Rather than coins, you need 3,000 Rune Arrows to recharge the Bow, and 20 Rune Kite Shields to recharge the Shield. Purchasing a new bow or shield costs 20% more (3,600 and 24).
 - Ava's Attractor no longer needs to be worn to retrieve your arrows, the effect is permanent once the Animal Magnetism quest is completed.