Available Online
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.
Meditations with Sri Nisargadatta Maharaj
Spirituality in a Nutshell
Master of Self-Realization
Kripalupanishad
Revealing the Secret
Yoga A Synthesis Of Psychology And Metaphysics
A Guide to Shaktipat
Devatma Shakit
Natural Yoga