Letter Quest: Grimm's Journey Remastered

Books are perks that can be bought from the merchant in Story mode and equipped to provide bonuses. There definitely aren't five worth purchasing, so once you have extra money and don't need anymore upgrades, buy a few books to unlock the achievement. Each book costs 500 gems.

It's highly recommended to purchase the book that increases the gems dropped by monsters first and equip it early, since it'll make your gem total rise much faster, allowing you to buy more. The other handy ones to buy are the one that deals more damage for 6 or more letter words, and one that heals you, since those two will help tremendously in battle. You can completely ignore the ones that affect the treasure chest minigame and the time trials, as they're useless (unlocking chests is easy and the time trials have way too much time as is).