Hidden Wisdom by Richard Smoley

Excellent introduction to mysticism in Judaism, Christianity, Islam, shamanism, Thelema, Freemasonry and the New Age movement. Not only tells you what each teaches but gives advice on finding a teacher in each tradition and good introductory books.

Jewish Meditation by Aryeh Kaplan

A guide to meditation in an Orthodox Jewish context. No New Ager Kabbalah here- Kaplan is one of Judaism's most respected scholars. The book shows how to turn traditional Jewish prayers into meditation as well as how to apply Bible passages for meditation. A must read for Jews, Christians, Muslims and probably everyone else.

Open Mind, Open Heart by Thomas Keating

A guide to a form of contemplative Christian prayer that has been passed down by the Catholic church for centuries. I actually just started reading this today but just experimenting with the exercises for less than 10 minutets, I am already impressed with how they can change my mood. I think this book could change your life.

Essential Sufism by James Fadiman and Robert Frager

Not a technique guide but rather a beautiful collection of stories, poems and teachings of different Sufi masters. A book that touches my heart whenever I read it.

Will add more as I get a chance.