The teachings of the Tibetan Master Djwal Khul through the Alice Bailey books are perhaps my personal favorites. She has written around 24 books and was among the first ones to talk about the New Age. If you have a 3rd ray or a 5th ray mind, the understanding of these books might become easy 😊. The profoundness of her books cannot be explained in words; however, need a lot of mental work and thus can be understood by those who have either lived or are living the experiences of the New Age. Her books carry some association with you no matter what stage of spiritual development you are at.
Even though her books are related to each other and have parts of every topic that humanity is dealing with at this time of the energy shift on Earth, if there is ONE book that I really have to choose, it would be Discipleship in the New Age Vol 1 and 2 as I could relate to a lot of my life events with the teachings in this one.
If you want to understand the concept of seven rays in depth, I highly recommend Esoteric Psychology Vol 1 and 2 and Rays and Initiations. Rays and Initiations also gives a great insight on the 14 rules of a disciple and initiate. It gives a clear understanding of the various initiations one undergoes on the path of identification with the Spirit.