Can you recommend some books that should be a part of any apologetics library?
There are certainly many good books from which to choose, so do not view these recommendations as being exclusive of other titles. But many would regard the following books as must-haves:
1) Radio Replies, by Frs. Rumble and Carty (TAN, 1979). This three-volume, question-and-answer format set (arranged topically) contains material taken from a radio show that was hosted by these two priests. The volumes cover over 4,500 questions that deal with virtually every topic that is likely to surface in any defense of the faith. The answers are succinct, informative, and extremely helpful.
2) Fundamentals of Catholic Dogma, by Ludwig Ott (TAN, 1974). This book is a thorough treatment of dogmatic theology (the Trinity, the Church, grace, salvation, the person of Christ, the sacraments, etc.). It provides the basis for and an explanation of these beliefs, drawing from Scripture, sacred Tradition, and the writings of the Church Fathers. It also indicates the theological grade of certainty for each teaching (i.e. certain, probable, commonly held, et cetera).
3) The Faith of the Early Fathers, by William A. Jurgens (Liturgical, 1970). This three-volume set provides extensive excerpts from the writings of the Church Fathers and ecclesial authors on a wide range of topics. This resource is particularly helpful in demonstrating that believers have held Catholic doctrines since the first century.