Here are a few lists of books by period, plus books on Christian iconography. The Find Books page (see tabs) will give you suggestions for to searching in more depth.
Most lists include books that can be requested through Summit
The Dictionary of Art is a great place to start. It is available in print only in the library on the Salem Campus.
St. Remy Basilica, Reims © Godong/Robert Harding World Imagery/Universal Images Group; Image from Britannica ImageQuest