Magic Book can be bought from the mall for 12 gold or 12,000 coupons or obtained (rarely) from a Chest or the Daily Spin. When you use the book you are awarded with a skill. You can assign this skill to any of your heroes. Each skill has its on unique property or boost. Once you assign said skill to a hero it cannot be unassigned, so choose wisely.
Said skills can also be upgraded through the use of an Upgrade Book, which costs 8 gold, and can also be obtained (rarely) from chest, daily spin, or even visiting.