Religious book shops used to be a feature of every small town. The local SPCK (Society For The Propagation Of Christian Knowledge) was an important shop where you could not only purchase religious books, but a full range of Penguin and Pelican paperbacks,, calendars, diaries, bookmarks, and a whole host of useful things.
Sadly the days of the High Street religious book shop are long past, but religious book shops can still be found, often attached to great churches and cathedrals. Havens of tranquility and learning these may be, but they also sell souvenirs, music, religious artifacts, tourist knick-knacks, and more, so the atmosphere is probably a little less scholarly than it may have been in the past.
I recall going to the book shop attached to Durham Cathedral and expecting to find a wide selection of books by the distinguished and world renowned theologian N.T. Wright, who was a member of the clergy there in recent times, only to be very disappointed that, on that day at least, they had almost none of his works for sale. There were plenty of rosaries, though, for some reason, which seemed slightly out of place in an Anglican cathedral.
A well-stocked religious book shop should appeal to a wide variety of tastes and beliefs. It should certainly have copies of the important religious books of all the major faiths – Islam, Hinduism, Sikhism, Judaism, Buddhism (although strictly speaking this is more of a philosophy than a religion), and of course Christianity.
In terms of Christian books, they should stock a range of Bibles in different translations, of which there are thousands. (In an ideal world, a bookstore could simply sell translations of the bible and nothing else!) Commentaries on the Bible, books by important Christian thinkers, theology, both scholarly and popular, Christian books for children, and these days even Christian novels should probably form the main part of the stock.
Christian gifts, such as rosaries, first communion gifts, crosses and crucifixes, greetings cards, and church supplies such as candles could equally find their place in the well-stocked religious book shop.