My short list of essential reading:

The Gift

Poems by Hafiz

Translation by Daniel Ladinsky

The Essential Rumi 

Translation by Coleman Barks

The Sivananda Companion to Yoga

(one of the best introduction to yoga books ever!)

Bhagavad Gita

Translated by Stephen Mitchell

Tao Te Ching

Translation by Stephen Mitchell

Light on Life

B.K.S. Iyengar

The Yoga Sutras of Patanjali

Translation by Swami Satchidandanda

Any and all of the books by:

J. Krishnamurti, Osho, and  Sri Arubindu

George Feurestein, an American scholar with many excellent books on Yoga