Let us carefully note the context of this quote and the way that this helps us to understand what it is precisely that Irving is saying through the location of Tom's Bible. As Tom so stridently declares that he has made no money from his usuring, the Devil comes to claim him, and we are given the following description:
Tom shrank back, but too late. He had left his little Bible at the bottom of his coat pocket, and his big Bible on the desk buried under the mortgage he was about to foreclose: Never was sinner taken more unawares.
There is tremendous symbolism in these lines. If we take the Bible as a symbol of our faith and relationship with God, Tom has forgotten his faith and relationship with God in exchange for pursuing greed and wealth alone, symbolised in the mortgage he is about to make money on. The ironic comment, "Never was sinner taken more unawares," is particularly true for Tom, as he is shown to have devoted all of his energies and efforts into selfishly accumulating temporary wealth rather than focusing on his eternal life.
Throughout the story Tom is only concerned with money. An example of this is when his wife disappears and Tom is only concerned because she took all their valuable with her. Tom only starts going to church and carrying a Bible at the end because he doesn't want to fulfill the deal he made with the Devil when he was given Kidd's treasure (giving the Devil his soul). By the big Bible being under the mortgage he was about to foreclose shows that Tom's new found religion is just a show and that he really is still just concerned with money.