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Here is a selection of books written by teachers from other traditions that I have some personal experience with, so I feel comfortable recommending them to others.  

Sri Ramakant Maharaj

Spirituality in a Nutshell

Sri Siddharameshwar Maharaj

Master of Self-Realization

Swami Shivom Tirtha

A Guide to Shaktipat

Swami Vishnu Tirtha

Devatma Shakit

Swami Yogeshwar Muni

Natural Yoga