I'm going to go out on a limb here and suggest a poet that you might not think (at first glance) would be relevant to good songwriting -- A.A. Milne. His poetry is not as simple as one might think, if you study it. It sings from the page and evokes experiences and imagery that both children and adults can understand and enjoy. Milne's poetry is a wonderful way to learn how to 'keep it simple,' which, to me, is the best way to write a song.
IMHO, the best songwriting is that which relates the writer's thoughts without making the words do fancy 'tricks'. For the best example of this, I'd suggest you study McGrath of Harlow's songs on his website. All kidding aside, Kevin is a master at making you think about the subject of his songs, and pleasing your ears at the same time.
All the best, Áine