Ian,
I don't think "safe" = "unchallenging".
The premise of the question was more about discernment. This is especially important in the life of a person who is a new believer or whose grasp of the foundations of Christianity is not yet solid.
When you pick up a book on Christianity, often, it strays from true Christian principles and foundation into a synthesized blend of Christianity, Buddhism, New Age, etc. How do you discern, prior to purchasing, that the book is true to biblical Christianity and not a synthesized blend? The person who lacks a solid foundation and who wants to follow traditional, biblical christianity would then be led astray with bad theology.
You're the Pagan Editor. Paganism originally was a term applied to polytheistic religion. In modern times, it has encompassed Wicca, etc. This is a stretch and may not be the best example, but let me try and relate it to that. If, as a pagan, you believed that the only true form of paganism centered around Greco-Roman polytheism, you wouldn't want a book that you purchased to learn more about it to be all about wiccan rites. Likewise, books on christianity are expected to be foundational and not a mixture of the new rhetoric coming out of the emergent church and other cirlces.
I am not against "challenging" books provided you know what you believe and why you believe it (have a foundation). I read books that express views in opposition to christianity. Going in, though, you know that is what you are reading.