-adjective 

odd or unnatural in shape, appearance, or character; fantastically ugly or absurd; bizarre.

Although, grotesque in art has been around for centuries, very little has been written in it's regard. Here is a selection of some books that deal with the topic specifically:

1. "Rabelais and His World", by Mikhail Bakhtin 

2 ."The Grotesque in Art and Literature", by Wolfgang Kayser

3. "The Grotesque", by Philip Thompson 

4. "Modern Art and the Grotesque", edited by Francis Connelly

5. "On the Grotesque", by Geoffrey Harpham

Know of any others? Please share!

* artwork by Joel-Peter Witkin

** Special thanks to professor Nancy Hightower for helping compile this list. Please check out her blog here.