Need advice on building a pious library.
Here are some tips:
1) Narrow down what you want: either by author or by type of book (Missal, Breviary, Bible commentary, prayer books, &c.).
2) Check book-selling websites (Amazon, Abebooks, Barnes & Noble, eBay) regularly with keyword searches (for example, for the works of St. Alphonsus, search "Alphonsus" and "Grimm" on Amazon or eBay). Sometimes booksellers may not know how valuable old Catholic books are, and you may stumble upon an English translation of the Roman Breviary for 15% of its market price amongst book collectors.
3) Check out thrift stores, flea markets, estate sales, yard sales, mom-'n-pop bookstores, antique stores, &c., every now and then. Also remember that Churches, Convents, Monasteries and Seminaries may be selling their old books.
4) It could be a lengthy process, but it will give you the opportunity to save some money.
5) It is now easier than ever to collect classic Catholic books. Reprographic reprints of these works are now being circulated and trustworthy translations of hitherto untranslated works are now available. One example of the latter are the lectures of St. Thomas Aquinas on the Epistles of St. Paul, which have the Latin and English texts. Also, the lectures of the same Angelic Doctor upon the Holy Gospel according to St. Matthew. It has been recently translated and published.
6) Universities are preserving deteriorating works from previous epochs by digitization and these can be downloaded as PDF files unto a sort of portable device that can enable you to have thousands of works at the tip of your fingers. Amongst these works are old Catholic books that are no longer in circulation.
7) Sometimes Chapels give or sell books that defunct individuals may have bequeathed unto individual Priests, whose private or Chapel libraries may have copies of these books or there may simply be no room for the books.
8) Pray to you Angel-Guardian, to St. Joseph and the Blessed Virgin Mary, that you may attain to the works that will help you work for the greater glory of God and the salvation of souls, particularly yours and those of your household.
I don't think of any other counsel to give you. If something occurs to me, I shall forthwith let you know.