Answer: What a fantastic question! First, let me commend you for being a supportive wife. There are sooooo many writers out there who wish their significant others would take even a slight interest in their work and provide the encouragement that so many writers need to keep going.
Second, let me ease your mind a bit. His writing does not mean that one day you're going to wake up to find body parts in deep regions of your freezer. Just as you would explain to your children (assuming you have some or are going to have some), that what they see on television is not real, what you read is not indicative of the person who wrote it. If your relationship was a completely normal one before you read the novel, then it will likely continue to be so. In other words, what's not in your freezer today will not be there tomorrow.
You wouldn't have been so shocked if it was written by an author you didn't know on a personal basis. You probably wouldn't have even given it another thought. But this is what writers do. We make up things (the key word being "make up"). Writers have an extraordinary talent for "picture memory". We can see the story play out in our minds and have a need to write that story down. Where do the ideas come from? It comes from everywhere, nowhere, our dreams, the environment... who knows? We just know that it's there and feel compelled to form that idea into a steady stream of words to create a story, a poem, a song... whatever the muse is to a writer.
You can calm down now and rest assured that your husband is still the man you know and love. And you should also take delight in the fact that if his novel was able to produce that kind of emotional response from you, then you have quite a talented writer on your hands.