Branch of philosophy that studies the nature of art

Reference

Aesthetics - Wikipedia, the free encyclopedia
"... Aesthetics also refers to the domain of philosophy that ponders beauty and art. An aesthetic (also esthetic or æsthetic) is the concept of a particular school of philosophy that appraises beauty and art by certain standards -- for example, 'he despised the aesthetic of minimalism'. ..."

Articles

FAQ: What does Objectivism Consider to be Art (Aesthetics), by William Thomas, 15 Mar 2002
"Many conventional accounts of aesthetics confuse decoration with art because they center aesthetics on the question of 'what is beauty?' ... because one's idea of beauty is inevitably informed and affected by one's sense of values, it is an issue that, like art in general, depends for its explanation on man's dependence on philosophical principles."

Books

The Romantic Manifesto: A Philosophy of Literature
    by Ayn Rand, 1969
Partial contents: The Psycho-Epistemology of Art - Philosophy and Sense of Life - Art and Sense of Life - Art and Cognition - Basic Principles of Literature - What is Romanticism? - The Esthetic Vacuum of Our Age - Bootleg Romanticism