This is the best book on yoga I've read so far. It has very detailed instructions for each asana, but more than that, it doesn't pretend that you can separate the asanas from the yamas and niyamas - in other words; you can't pretend that yoga is just another kind of exercise and actually get much out of it.
He is a teacher in the tradition of Integral Yoga, a spiritual student of Swami Shivananda, who in turn seeks his inspiraton from Patanjali. As do most. The difference is that instead of preaching a particular branch of Hinduism, a particular path, Shivananda and his own forefathers believe, as Krishna himself laid out in the Gitas, that all four paths can and should be practiced together - thus the "integrated" title.
Van Lysebeth does not get preachy, though. He just lays out what one needs to do in order to get the most benefit from practicing yoga. I highly recommend it, even for those who are more experienced in the physical practice.