This was a formative works for me growing up. It opened my perception of what poetry could be after a being exposed to and writing more traditional metered and rhyming poetry.
Leaves of Grass by Walt Whitman. http://www.amazon.com/Leaves-Grass-Original-Edition-Editions/dp/0486456765 (There are a lot of renditions; some short, some longer. Doesn't really matter which you read initially, I think.)