Aqueous coating is a newer and evolving coating. It’s applied wet and looks like a bucket of diluted white glue sitting next to the press. It’s relatively inexpensive, offers good gloss and better protection against scuffing than varnish. More presses today are built with a final tower to coat the sheets in-line with aqueous coating so there’s no additional press time to charge for and no plate to make if you’re flood coating the sheets.
Be sure to ask your printer for samples of an aqueous coated cover before you specify it for your book. Fold the outside covers against themselves and rub them against each other face to face. There will be scratches in the coating but the ink should be intact. It will be up to you to decide if the scratches are serious enough to limit the sales of your book or trigger an avalanche of returns. It’s a great cost cutter for journals and directories where the appearance of the cover isn’t quite as critical.