books on openings
Probably you want an encyclopedia type book that you can look up the main ideas for every new opening you come across. Paul van der Sterren's Fundamental Chess Openings is pretty much the gold standard because of his clear explanations of why moves are made. Later, probably much later, when you have settled on a few openings to use, you may want to go deeper. And of course as a diamond member, you already have access to tons of training material in the Lessons and Videos. Have fun!