I recommend Sproul's Are We Together? first; then De Chirico's book mentioned by Phil above, then Allison's book Roman Catholicism. I do NOT like Boettner's book at all, and not just for reasons of being published pre-Vatican II. He took mystics of the Roman Catholic church as the standard-bearers for it, instead of the Baltimore Catechism and Ludwig Ott. He gets quite a few facts incorrect, and, like so many others in the evangelical world, atomizes Roman Catholicism instead of treating it like a system (this is the substance of De Chirico's outstanding work). Ott is an excellent representative of Thomistic pre-Vatican II theology. As mentioned above, Denzinger's Enchiridion is essential original source material. Get the latest edition (43rd edition) in order to have access to lots of recent bulls and encyclicals. Understanding Catholicism today means understanding the impact of Vatican II and the controversy over its interpretation. To understand the impact of Vatican II, I recommend the following: What Happened at Vatican II? by John O'Malley, and Vatican II: Renewal Within Tradition, edited by Matthew Lamb and Matthew Levering, which will give you both sides of the debate today.