So your spouse wants to write/is writing/can't (or won't) stop writing a book.
First of all, STAY CALM. There is hope. (Well, not really, but what kind of survival guide would this be if I said things were hopeless?) While there might not be a cure, you can relieve some of the symptoms if you play your cards right.
Step 1: Monitor the situation carefully. Are they talking to themselves? Refusing to wear anything besides pajamas? It will be okay. This is all very normal behavior for novelists. It's even normal for them to Google ways to kill without leaving a trace, so don't take it personally. You're probably fine to still sleep with them at night, unless that is, you are bothered by their constantly getting up to write down new plot ideas at all hours.
However, if they start crying over every rejection, or going into fits of rage whenever another big name celebrity gets a book deal, it's probably best to move to the next stage...
2. Chocolate. Chocolate is always the answer. If they ask whether their critique partner was right about their weak prose, simply shrug your shoulders and offer chocolate. If they spend an hour looking at the blinking cursor on their screen without writing a single word, offer chocolate. Bad review? Chocolate. Twitter pitch contest? Chocolate. It's always chocolate.
Step 3: Learn some basic publishing terms so you can follow along when your spouse starts going off on the "state of publishing" or "indie vs. traditional" or any other host of things that are likely to spark a fifteen minute monologue. (Be prepared, these monologues can sometimes be accompanied by tears, so have a tissue handy).
Step 4: Practice phrases like, "No, it's okay, I didn't really want dinner tonight", and "I think yoga pants are sexy," and "Who cares if the house is a pigsty?". These will come in handy more times than you can count.
Step 5: Get used to it. Face the facts. Writing is an incurable disease, so you might as well get on board. I hope you enjoy crazy!