Many of the following books can be found for free on the internet, in electronic versions, no longer bound by copyright. Just search and you will probably find many different sites that host these texts as free downloads (in different formats, for example, as text, PDF, mobi, epub, AZW, etc).
If you prefer a paper copy, trying searching an online bookseller, such as Amazon. Older, more obscure works might be available at BNZ Enterprises Catholic Books.
Newer books, still bound by copyright, can usually be found both in paper versions and electronic versions at an online bookseller.
The following web pages each have lists of books that are recommended, in a wide variety of topics, for your continuing formation:
Knights of Columbus: Catholic Information Service
St. Paul Evangelization Institute: Books for Catholic Evangelists
John Senior's Incomplete Life-long Reading List (ages 2 to adult): Will Rascals Defend Our Civilization…and What Books will they Read?
Brandon Vogt: Best Catholic Books of All Time
Brandon Vogt: More of the Best Catholic Books
Catholic Answers: The Ultimate Apologist's Reading List
Fr. Gary Coulter: Recommended Books to Read (lots and lots, of different genres)
updated 7-29-19/jwk/ls