Emptiness
I'm reading The Tantric Mysticism of Tibet by John Blofeld right now, along with Eye to Eye by Ken Wilber. I love both of these books. The subtitle of the Eye to Eye is The Quest for the New Paradigm, and it certainly does the job.
But back to Blofeld for a minute. Last night I read a line about visualization, particularly the tantric practice of becoming the deity that you are visualizing. Blofeld contextualized this by saying that visualization is something that is quite foreign to the western mind and that usually the idea of manifesting reality is seen as hocus pocus. But he says the idea is really based on the concept of emptiness, that the meditators emptiness and the deities emptiness are one and so that is why it is possible to become that visualized deity. I just found that one statement to very profound, put the book down and started meditating!