For Christianity I highly recommend Saint Augustine’s Confessions. It is very readable and the author’s intellect shines throughout. Edit: I hear very good thi

For Christianity I highly recommend Saint Augustine’s Confessions. It is very readable and the author’s intellect shines throughout. Edit: I hear very good things about C.S. Lewis's Mere Christianity too, but I've never read it. Also there's always Aristotle. Thanks to him we know there is not really a clear line between theology and philosophy. It's fascinating to me that an ancient Greek pagan derived a kind of classical monotheism as being philosophically necessary. One may disagree, but I don't think anyone seriously interested in the subject denies the importance of his contributions. Others have already suggested the various holy and foundational texts as being worth reading. I'm inclined to agree on account of for thousands of years before printing when keeping a book extant was extremely laborious people from every culture chose some writings as being particularly precious. Surely they had what seemed to them at least to be good reason. And casting a broader net, reading the myths and stories of the religions can give you a good feel for how believers see it. I'm mostly familiar with Viking and Greek myth, but there is a rich tradition of mythology in the east too. I've never made a serious study of the Vedic stories, but I've read a few adaptations and there is a lot of interest there too.