Atheism is, indeed, a respectable philosophical stance. The Cambridge Companion to Atheism would provide a helpful overview, and for contemporary reflection on atheism you might check out the Oxford University Press book Philosophers Without God, edited by a panelist on this website. Michael Martin's book Atheism is a massive sustained argument for atheism. There are, indeed, many positive philosophical works on theism and Christianity in particular. I happen to be a Christian philosopher (or a philosopher who is a Christian) so I do not think you have been buying the wrong books! But I study and engage with the excellent and growing body of philosophical works that advance atheistic positions and I recommend these to you.
Somewhat related to your question, please allow me to add an observation about the concept or category of *atheism.* English usage of the term may be somewhat fluid, but I am inclined to think that if someone (a philosopher or professor in some other field or, really, anyone at all) has never given any serious thought about theism (the belief that God exists), then it might be more accurate to think of them as *non-theists* rather than atheists. The reason for this is that the term "atheism" suggests (or so I suggest) a rejection of theism which (again, in my mind) suggests theism has been entertained. An analogy might be this: imagine a philosopher who has never really thought about Hegel. It would be odd to think of her as anti-Hegelian, but it would not be odd to think of a philosopher like Kierkegaard who studied Hegel's work closely and rejected it as anti-Hegelian. In terms of those philosophers who self-identify as atheists, there is a wide spectrum of viewpoints. One philosopher, William Rowe, who is an excellent philosopher (widely recognized as excellent by both atheists and theists), describes himself as a "friendly atheist." You can do a search for the term and find some interesting references on the web. What Rowe meant was not so much that he would (for example) gladly buy you a drink whether or not you are an atheist (on this matter, I can't say as I have never met him), but he means that while he thinks that atheism is true, he believes that atheism is not the only option for rational persons. He believes, for example, that while the problem of evil provides a person with a good reason for thinking atheism is true, a person can reasonably think that the existence and quantity of evil is compatible with believing in an all good God.