Robert Assagioli wrote an incredible little book about will, long before neuroscience gave us The Power of Habit or The Willpower Instinct.

The best thing about studying Assagioli is that it isn't difficult to read his writing. Its theme can be overwhelming; but the map to a coherent existence that it offers is priceless. Some of us yearn for a deeper existence and a life of meaning must be pursued consciously. By understanding the qualities of will, the types of will, and using will to harmonize our sensations, feelings, impulses, imagination, thoughts, and intuition we can lift ourselves to a higher realm of consciousness.

Assagioli's Relationship Between Self, Will and Other Psychological Functions