http://www.amazon.com/Broken-Thing-Poets-Line/dp/1609380541/ref=sr_1_1?s=books&ie=UTF8&qid=1321986713&sr=1-1
Why does the poet stop the line (line break) when he or she does? It is not a simple decision since the 20th century arrived and poets  began to experiment with more than traditional    blank verse that consisted of repeated  predictable  patterns of stressed and unstressed syllables.
Where does a line end or begin  in a poem? Its intent, content and form is not the  same as a line in prose.  Reading A BROKEN THING- POETS ON THE LINE is like falling into an alternate universe where words,  sounds,  pacing, silence, page margins, enjambment, compression, wide  open spaces all take on  a powerful vibrant life of  their own.  Political views can even be seen in the construction of the line! A poem  is not a static thing. The variation of  structure are endless.
Think about this-
When a line ends there is  often silence. What does that silence do? Is the empty space in  a line soundless?
In this book of essays poets tell  readers how they see the line and how they use it in their work  and how others use it.
I particularly enjoyed these essays:
Who is Flying this Plane?
 The prose poem and the life of the line by Hadara Bar-Nadua.
Croon: A Brief on the Line by Tim Seibles
Three Takes on the Line by Catherine Barnett
This is an anthology you can read again and again.
Read more about blank verse (the traditional poetic form) here
http://www.uni.edu/~gotera/CraftOfPoetry/blankverse.html