Nowadays, of course, books means not only printed books, but audio books and ebooks. Here are three places to find Christian books of all formats. The top two also have much more Christian-oriented products than just books.
I have been delighted with Christian Book Distributor’s (or Christianbook.com online) excellent selection and service for 30 years now. All of the study materials I use to write my posts have come from them, and I might not have been able to find some things anywhere else. Besides books (from esoteric to best-sellers) they have printed and recorded music, greeting cards, calendars (in season), church supplies, gift items, and more.
Christianbook.com does not carry out-of-print items. Here are my suggestions:
Everyone knows Amazon. You can find closeouts and used books, among other things. But check one other place first.
Book Closeouts specializes in out-of-print and overstock books. Compare its prices with Amazon. You’ll probably save money that way. If you don’t mind minor dings and scratches, you might be able to save even more. I have also seen books for sale here autographed by the author.