I’ve just finished Sam Harris’ book Waking Up: A Guide to Spirituality without Religion. I highly recommend it, as it contains important insights both for materialists who would normally dismiss meditation/contemplation and for supernaturalists who mistakenly think that their inner experiences furnish evidence for metaphysical truths.
Harris also points out the logical flaws in using such inner experiences to justify metaphysical claims, exploring the evidence that indicates that minds arise from brains and investigating how physical changes to brains – including during near-death experiences and drug trips – can cause experiences that resemble trances that meditation can induce more gradually and safely. He also discusses how unscrupulous gurus can take advantage of people and justify this behavior with some of the insights generated by meditation.
Some of the ideas covered in this book – and a short guided meditation – are presented in a very interesting talk Harris gave about “Death and the Present Moment,” which you can view here.