It’s a strange phenomenon: as seafood has become expensive and endangered, more fish cookbooks seem to be coming out. Maybe it’s because people want to make the most of their increasingly limited choices. Or maybe authors want to memorialize dishes that may soon be memories. The latest is an offering from Barton Seaver, a Washington, D.C. chef, TV host, and sustainability advocate. “For Cod and Country’’ is Seaver’s first book, and it has a little of that translated-from-restaurantese quality that one often sees in chefs’ first books. Recipes evince a bias toward plating and presentation, and a certain disregard for the number of pots you might have to go through on the way. Still, the book offers some bold, thoughtful flavor constructions.