This is not about poetry, not directly anyway. Rather it's about where I find stimulating reading not necessarily for the summer. I direct you to the UC Berkeley summer reading list for incoming freshmen. I've found delightful reading from these lists over the years-- Snow Crash by Neal Stephenson is one example I remember reading while vacationing on the National Seashore, Cape Cod several years ago. It remains a remarkable reading experience. The books on Berkeley's list say a lot about what incoming freshmen might be expected to know. I hope the promise plays out in reality.
I think too that a Google search may turn up older versions of the list so you may have a lot of choices and will find something palatable, exciting and stimulating. In any event reading some of the books will ultimately enrich your poetry as all good learning will.
While on the topic of reading lists I now call to mind Alan Ginsberg's "Celestial Homework" reading list from a course he taught at Naropa in 1977. If I could I'd read all of it. The link takes you to the list which in turn has links to some of the works online. It's a marvelous way to find some grand and great writing and also a look into Ginsberg's own makeup, that of one whom The New Republic magazine (I think) referred to as a "major minor poet".
In all events, if you want to write, read!