I loved the response from someone who suggested you take your wife fishing, as that would disorient her! I'd love for my man to drag me off to go fishing, but.....
On a more serious note, the issue is not with Harry Potter per se, but with fan fiction and the fact that this really appears to be an obsession. Is she only reading the fan fiction, or is she writing it, too? Have you been able to share your feelings with her, using "I-messages" (i.e. no blame) when she is more approachable?
If she is writing fan fiction, ask her if you can read some of what she has written. (This could go beautifully or it could go over like a lead balloon.) Sometimes our partners will have a hobby that we just don't like, but if we feign some interest, we have opened a new door of communication. If she is a good writer, encourage her to hone her craft on original works. Offer to help her find information on publishing.
If she is writing and does let you read what she has posted on line, and if her writing is among the smuttier fan fics you have seen, you should heed the advice someone else mentioned about marriage counseling. If she is resistant, identify a short coming or two of your own that you want to work on in counseling, so it's not all about her & the fan fiction. It's an investment in your future that may be expensive or take some time, but is well worth it. There are sliding scale agencies if you don't have insurance.
As you are realizing, the number of hours is a serious concern and even more alarming when you mention that she doesn't do any chores....does she have a stressful job outside the home?
Is this her version of the couple of drinks after work some people use to medicate the stress they are under? (Not that this is a good approach.) Are chores fairly divided within the home? Does she keep up with other activities (sports, hobbies, crafts, etc.) and friendships she has enjoyed in the past? Has her obsession with fan fiction interfered with her professional life or your family's finances?
Not to be an alarmist, but if she has pulled away from people and activities she used to enjoy, I would also be concerned about depression. From the point of view of one who has had to seek assitance for different things at different points in my life, it can be scary. As you work up the courage to suggest counseling, try to detach from her obsession with love. Do not detach from her, but from her obsession. (Go to websites with information on Alanon, which is a support group for friends & relatives of alcoholics. The tools they use are applicable no matter what the
addiction, obsession, or stressor is.)
Also, you sound like you probably have been wise enough to know that nagging will not help. Are there any activities you once enjoyed, but have been putting on a back burner? Perhaps getting back into something that was fun for you would help fill some of the inevitable void you must be experiencing. You might also start keeping a (discreet) journal or log of how much time she is spending because she probably isn't aware. Don't rub this in her face, but tuck it away as objective evidence that might be gently used if / as / when you are able to convince her to go for some professional help. And if she won't go, you go yourself.
My guess is she has no clue how much pain this must be causing you, and if she were able to really understand, she'd probably feel terrible about it. Addictions and compulsions are
diseases, treatable diseases. She has a disease, but she is NOT a disease. If she had cancer, you wouldn't treat her differently, and as hard as this sounds, the more compassion you can have for her AND for yourself, the better.
I wish you well. I bet you are not alone, either. What you must be going through must be similar to someone whose spouse has workaholism, a gambling addiction, etc. Get the help and support you need, and even if she can't see that she has a problem (which tends to be proof that she does have one) I hope you keep getting well and taking care of yourself. Good luck!