“Dragonflying is good for jaded birdwatchers. It presents new challenges,” Cynthia Berger told me as we watched darting dragonflies at Whipple Dam State Park one sunny day in late July. Berger is the author of Dragonflies, an excellent new book designed for beginning dragonfly-watchers. These “glittering aerial acrobats,” Berger writes in her book, are similar…