Aristotle, Dante, Sidney, Shelley

Poetics
http://www.leeds.ac.uk/classics/resources/poetics/poettran.htm

Letter to Can Grande
http://www.english.udel.edu/dean/cangrand.html

An Apology for Poetry
http://en.wikisource.org/wiki/The_Defense_of_Poesy
Or
http://www.bartleby.com/27/1.html

A Defence of Poetry
http://www.bartleby.com/27/23.html