Great suggestions above! I loved When The Wind Stops; Charlotte Zolotow's books are really thoughtful. And we read McCloskey often.
To add to the list, I highly recommend The Curious Garden by Peter Brown. It's a newer book about people's love of gardening and connection with plants, with a great permie note about weeds as pioneers rehabbing lifeless spaces (an old, elevated railway track). It's based on the true story of the Highline park in NYC, which you can visit.
For even younger kids, we loved Potato Joe by Keith Baker (not that profound but Really catchy); Baker's Little Green, with a caterpillar to find on each page; and A Good Day by Kevin Henkes. Beautiful Oops by Barney Saltzberg has a great message about finding creative uses for the things around you, even if you initially see them as mistakes. We also really liked Maple Syrup Season by Ann Purmell and, later, Life Story by Virginia Lee Burton.