In the De Controversiis, St. Robert Bellarmine defends the doctrines and teaching of the Church against all comers, starting from Scripture, the Church Fathers and also reason. His work was widely read and commented on by both Catholics and Protestants and quickly became one of the standard texts in Catholic theology for centuries.
In On Purgatory, Bellarmine defends what is one of the more difficult doctrines to understand in his characteristic style beginning with Scripture and the Fathers, stopping at every step of the way to answer the objections of all the major Protestants of his day, not only Luther and Calvin, but also those less known to us such as Brenz and Peter Martyr.
Dividing his work into two books, Bellarmine shows that there is such a place as Purgatory by copious exegesis on Old and New Testament passages, and the clear consensus of the Church Fathers who witness the fact that prayer was made for the dead in the early Church.
Then, in book 2, he examines questions about the specifics of Purgatory, what souls there suffer, where it is located, how the faithful can assist the souls of Purgatory, and other questions.
This treatise, translated into English for the first time, is the best and most in depth treatise on this subject available, and is just as relevant today as when it was first penned.