Via the blog Hieroi Logoi, I learned that the catalog from a Metropolitan Museum of Art exhibit of Late Antique Christian art in 1970s, with the title Age of spirituality: Late antique and early Christian art, third to seventh century
, is available as a free pdf download. It can also be read online.
The book is out of print, and a used copy on Amazon.com is selling for $349.88. I think the free digital copy is better value for money, even though the print copy is only about $.50/page, since the book is 737 pages in length.