The various props in A Doll's House have great symbolic importance. In the early stages of the play, they're used primarily to represent the ostensibly happy, contented married life that Nora and Torvald present to the outside world. The beautifully-bound books that line the walls of this cosy family home are suggestive of middle-class respectability. Before we've had time to be properly introduced to any of the characters, we are already aware of what kind of environment they live in. None of this gives us any clues as to the true state of this sham of a marriage.
The books, like the other props in act 1—such as the Christmas tree, the stove, the fire, and the candles—set the scene for what is to follow. In their suggestions of warmth, comfort, and stability, they provide a deliberate contrast to the true nature of the Helmers' relationship. The subsequent shocking revelations of what's really going on beneath the surface are made all the more effective for having taken place against the backdrop of such a conventional bourgeois home, with its bookshelves groaning under the weight of impressively-bound tomes.