The best things about summer are the long days outside, in a yard, in a park, by a pool, on a patio, in a hammock, or someplace naturific, and having all the daylight in the world to get lost in a good book. The term "summer reading" takes on a vastly different meaning once you're done with school and reading lists, and tends to be synonymous with guilty pleasures. I usually take advantage of the long days and cool nights to tear through the teen novels that more productive seasons push aside.
Another nice thing about summer reading is the "dipping". Sometimes you don't want the sustained attention of a novel or non-fiction title, but you just want to dip into something until the sun lulls you into a nap-like existence. That's where poetry can come in handy. The volumes tend to be slim, the stanzas or passages short, and if you let a book fall open at any old page you're on the right track.
For several years, Leaves of Grass has been my favorite book to pack for camping trips or day hikes. I'm still not sure if I've read the entire thing cover to cover, but it's a great companion in the great outdoors. If smoke or heat or fingers-crossed monsoons are keeping you mostly town-bound this summer, check out The Walt Whitman Archive, which links to six American editions of this tome, as well as a scanned reproduction of Walt's own 1860 copy. Dip in as needed, and let the language lull you into the long summer days.