The first two are obvious choices: the Catechism, and the Summa Theologica. The Catechism is written more as a confession of Faith rather than an apologia, but it's a great starting place for determining precisely what the Church teaches. Plus, if you hunt the footnotes, you'll find lots of references to papal encyclicals which explain each topic in greater depth. The "Summa" is almost unreal. Thomas presently the strongest form of the anti-Catholic argument for each proposition (providing quotes which seem to support each position), then dismantles each one systematically. If you're a Classics major, you may have already read him. If not, do so. That said, the Summa is crazy long, so you might consider New Advent's online wiki-style version, so you can find specific answers to specific questions.
Msgr. Ronald Knox was a genius and a classist to boot. When I say "genius," I mean he was writing Latin and Greek epigrams from the age of 10, was a widely-read author by his twenties, and single-handed translated the Vulgate into English. He wrote a lay-friendly book called "The Belief of Catholics" which remains one of the finest short systematic treatments of Catholicism I've ever seen. As much as I love "Mere Christianity" (and think Lewis may be a more engaging writer), Knox is a more rigorous thinker, and predicts (and refutes) virtually every counter-argument you can come up with. [I've only read one of his other books ("The Church on Earth: The Nature and Authority of the Catholic Church"), and while it wasn't nearly as engaging, it was very engaging. So Msgr. Knox is probably a good author to check out overall.]
Fourth, I'd say "Against Heresies" by St. Irenaeus. He's writing it to instruct new Catholics in the Faith, so it's naturally systematic in scope, and explains lots of things which many other Church Fathers just assume their readers know (like that the Church is subject to the Bishop of Rome, that Apostolic Succession is the mark of the True Church, that the Liturgy is a Sacrifice, that the Eucharist is the Body and Blood of Christ, etc.).
Fifth, I'd say St. Francis De Sales' "Catholic Controversy". He's better known for his "Introduction to the Devout Life" (an incredible devotional book for those interested in improving their spiritual lives), but "Controversy" takes on the specific issues dividing Catholics and Protestants, and shows why the Catholic position is correct.
Hope that helps. If you're interested, I run a Catholic blog, too (www.catholicdefense.blogspot.com) and am more than happy to field any specific questions you've got about the Faith. God bless!
Joe.
P.S. Here's a bonus book: Fr. James O'Connor's "The Hidden Manna" deals specifically with the Eucharist, but the style is one you'll likely find appealing. He goes chronologically through the writings of virtually every major Christian thinker. Part I is on the Early Church on up through the Middle Ages, Part II deals with the Reformation, and Part III deals with the modern era. Part I is by far the most interesting. Fr. O'Connor's writing style is engaging, and he explains ambiguities and likely meanings in the Greek in an easy to understand way. He saves a lot of technical stuff for the footnotes. The clear conclusion one draws after reading is that the early Church was unanimous in its belief in the Eucharist. This one is technically not about "all" things Catholic, but it incorporates collaterally many other important things Catholic (like that there are bishops in the early Church, that people are answering to Rome, etc.).
I'm curious: what books should I have included? What do your top fives look like? (If you can't think of five, that's fine: just mention whichever ones you find are your personal favorites).
