I have a theological reference library that cost me thousands of dollars, which mostly gathers dust, thanks to the Internet. Even my Bible is seldom opened, because I can more quickly find what I’m looking for online (in English, Hebrew, Greek, and 40 translations, cross-referenced with multiple dictionaries, lexicons and commentaries). My massive Oxford Dictionary of the Christian Church, for which I paid $300 (really), which was once always on my desk, has not been opened in years.
The one reference that remains always on my desk is William Jurgens three-volume set, The Faith of the Early Fathers. It has exerepts from hundreds of early Church documents, but - best of all - it has a doctrinal index linking to the passages. Want to know what the Early Fathers wrote about baptism by heretics? It’s there. And, by the way, Jurgens cites passages which both support and oppose the doctrine (the Fathers were not unanimous, and most of them opposed the particular doctrine I cited).
What I have said applies to reference books (books that you use to look up stuff, but not just read). There are many great books worthy of reading cover to cover, which Google will never replace. I could name several, but I will name just one, to illustrate how invaluable I think it is. It is The Lamb’s Supper: The Mass as Heaven on Earth by Dr. Scott Hahn. This was a gift from my brother (a priest), and it absolutely blew me away. My wife read it, and it absolutely blew her away. No book has ever astonished and enlightened me as this book did.