1. The bible is essential, anthing else you read should be on top of the bible
2. Jeanne Marie Bouvier de la Motte Guyon A method for prayer
3. Herein is Love: A Study of the Biblical Doctrine of Love in Its Bearing on Personality, -Reuel Howe
4. Thomas a Kempis - The imitation of Christ
5 Charles Ebert Orr How to Live a Holy Life
6. Marcus Dods, 1834-1909. How to become like Christ
7. Release the Power of Prayer by George Muller
most of these books are available free on the internet here
https://www.gutenberg.org/wiki/Christianity_(Bookshelf)
or as audiobooks on youtube