So it's not just that books are everywhere in their home, as just another component of its arguably contradictory warmth and color. Books are everything; they hold up the kitchen countertops and constitute the walls. But the family doesn't think this is an arid, emotionally barren way of life, particularly not dad Christopher, who doesn't admit the validity of any thoughts or feelings that can't be put into words -- ideally the sort you can shout back and forth simultaneously and ignore when convenient. They think it's a normal, wonderful way to be, and Billy's parents chose long ago not to have him learn sign language (and, significantly, not to learn it themselves into the bargain).
Back to that cohabitation. Billy's just finished university and he's only recently moved back home for the time being. His older brother and sister happen to live at home as well, but they're no longer pleased to be there, getting constantly under each other's skins as well as their parents.' Throughout Billy's life, he's understood what he can catch through adroit lip-reading, and he's learned to speak clearly with the help of hearing aids, but we learn in the play's opening dinner scene that he isn't really included in the perpetual barrage of jokes and insults that fly among the other four, and he never has been. (I might be grateful for that, but not for having been denied the option to choose it for myself.)
Into this obviously precarious situation is dropped Sylvia, Billy's first girlfriend, who signs fluently because she was born to deaf parents but is now gradually losing her own hearing. Soon she'll be giving up her passport to the hearing community, and as if she didn't have enough on her plate, she gets to meet Billy's clueless, confrontational family, which dissects her like a rare bug. When Billy realizes what a large world of communication is available beyond his walls, he struggles with how to appreciate his new world without alienating his family or disregarding Sylvia's loss and depression.