I'm reading St. Thomas Aquinas' Summa Theologia. This is a very heavy read, originally written in Latin by the scholarly Thomas Aquinas who was a Dominican monk living in the 11th century. I've been taking it slowly, which allows me to absorb the content, since it's almost as hard as reading Shakespeare. I enjoy it though, and have learned a fair bit from what I have read so far. It's a huge work, and will likely take me years to finish, but I figured I should read it sometime, since it's so well known and frequently referenced in Catholic writings/Catechisms, etc. There are some things in it that are inaccurate, but considering Aquinas was writing a thousand years ago, I give him a fair bit of slack in those areas.

I don't recommend this to people who aren't heavily interested in Catholic theology, but for those who are...well, you might enjoy it if you have the time to work through it, but his "Shorter Summa" (I'm not sure, but I believe it is called just that) may be a more preferable option.