A tale of programming and software best practices from the Computational Fairy Tales universe. In all his years as a wizard, Marcus has never seen a spell cause this much damage. When Hannaldous's sloppy attempt at a shield spell accidentally curses the castle, the walls start crumbling at an alarming rate. Now Marcus and his apprentice Shelly must figure out how to repair the damage before the castle turns to dust. Along the way they will encounter gossiping worms, perfectionist bakers, opportunistic rabbits, and copious amounts of mold. The Best Practices of Spell Design introduces practical aspects of software development that are often learned through painful experience. Through Marcus and Shelly's quest, the story encourages readers to think about how to write readable, well-tested and maintainable programs. Readers will discover the importance of comments in recipes, the value of testing potions, the dangers of poorly named ingredients, the wonders of code reviews in magic libraries, and the perils of premature optimization.