Most resources you find will have origins in Buddhism. Ignore it. I'm an atheist too, and while I can't speak to the link above, look up Mindfulness in Plain English for another source that, while written by a Buddhist, is pretty much an "instruction manual" where you can just ignore every mention of spirituality and Buddhism (there aren't many of them beyond giving some context in the introduction). While I groaned once or twice (a mention of levitation...), the spiritual/religious content in that one is light enough that it should be easily tolerable for all but the most rabid hardline atheists.
It's available for free at urbandharma.org (there's a direct link elsewhere in this thread, that I'm too lazy to look up) or you can get a hardcopy from Amazon.
It's not. There's a brief strange bit where he thinks advanced meditation skills can lead to psychic powers, without really distinguishing them from people's perceptions of reality changing. Other than that it's pretty much "do these exercises with your head and look for this stuff to happen", and some ranting about general meditation instruction not being enough of that.
The book is written by a Western MD and focuses heavily on results. It leans on the classical maps of the path of meditation from Burmese Theravada, a tradition that is about as stripped down and non-theistic as it gets. That said, the maps and some of the terms used may appear somewhat archaic to a modern reader.
If I understand the Buddhist teachings correctly (and I may not) there is no ‘religion’ here in the sense of a theology, afterlife, deity, etc. I guess we can define religion in a number of ways but those characteristics seem to me to be essential elements.
