My friend Jonas Salzgeber recently commented on the phenomenon where we take the time to learn things that will improve our lives, yet fail to properly apply them. In my experience, a major obstacle to putting what we have learned into practice is information overload and it's accompanying disorganization (not just the force of old habit or the lack of will to change - although these are major obstacles too!) Without proper organization and memorization, what sticks in our head after reading a good book or watching an interesting video is piecemeal. It's a matter of chance whether we remember and apply it, despite our best intentions.
The ancient schools of philosophy had their own solution to this problem - handbooks. These were collections of useful and powerful phrases that the students memorized and kept close "at hand," for when they were most needed - in instances of challenge, stress, or trouble. As we'll see, the handbook is the most powerful tool in our arsenal for growth - both an organizational aid for our hectic lives, and a method for directly improving our habits and thinking.