A dictionary of art historians is a big book that tells us about people who have studied and learned a lot about art. Just like we go to school to learn about different subjects, some people go to school to learn everything they can about art! These people are called art historians.
The book has lots and lots of entries, or short stories, about each art historian. Each entry tells us about where the person is from, what they studied, and what they know a lot about in art. Think of it like a really big list of your friends, but instead of just their name and what they like to do, it has a lot more information about what they're really good at.
So if you're interested in art and you want to know who the experts are, you can open up the dictionary of art historians and learn about all the amazing people who have studied and taught about art for a very long time!