After completing a few levels you will receive the option to purchase and equip books. Hit
from the games level map and
once to select a book. It costs 500 gems to purchase your first book. At first, you can only equip one at a time but later in the game you gain the ability to equip up to 3 simultaneously. Every level you complete increases book XP for the book you currently have equipped. Just choose a book that works best for your play style and complete games. This achievement will come early wayyyy before completing the game and will come naturally while working on other achievements.
(edit: if you pay attention to the stats of each monster you fight, on the lower right corner of the screen you will see a book icon with a number, this tells you just how much book xp you will receive for this fight)