First off, my own anecdotal findings are that atheists are some of the best read people on religious subject matters. So I would say many of them have no qualms

First off, my own anecdotal findings are that atheists are some of the best read people on religious subject matters. So I would say many of them have no qualms with reading religious themed work. Secondly, at its core Zen Buddhism is more philosophy than religion. I'm an atheist and can still agree with most of the concepts found in Zen Buddhism. It's not about worshipping gods but more about the philosophical teachings of Siddhartha who most Zen Buddhist regards as having no particular supernatural powers. I would hope most non religious people have no bias against it and if they do I would recommend they give that a second thought.