I just had a major brainstorm. I was reading American Primitive, Mary Oliver's early poetry book, and thinking about how these poems differed from her more recent work. I noticed that this book, which won the Pulitzer Prize, was different from her later collections. Apparently, you can reach the peak of success and still make radical redirections in your writing.
It occurred to me (duh! am I the last poet on the planet to get this?) that it would be instructive to study a poet's work chronologically, to follow her/his poetic development by noticing what elements they tossed out. In Oliver's case, it was social commentary. Increasingly, the social/political context disappears from her poetry, as do other people. But in American Primitive, these elements occurred often.
I'm going to try this with Marianne Moore, whose oeuvre is large and so, hopefully, will show even more rigorous exercise of choice.
In poetry, the most interesting thing is often what's left out.