You see, those of us with family members on the autism spectrum get dragged into their atypical relationship with books. We "must" go regularly to the library, where they borrow every book on their preferred topic that they can find. Sometimes, they borrow the same books over and over again (or "convince" their siblings to get the book out for them, since they each only get three books). We buy our Aspy relatives books about their favorite topic as gifts because they're guaranteed to please.
Sometimes, access to these favorite books has to be monitored, controlled, or even doled out as a prize because our kid wants to spend all day reading about space, cowboys, trains, or the like. They'll forget to eat, put off bedtime indefinitely, get distracted away from their homework if you don't pry their prized books out of their hands.
At one point, I had to complete a survey. It read, "Does your family choose books, outings, and vacations based on the autistic family member's special interests?" Well, yeah. The neurotypical family members read the books favored by our family members with autism just so we can make conversation. Eventually, it may even become a genuine interest of the other family members. Or at least we know a lot of stuff about a lot of stuff. Like space, cowboys, trains, or the like. (And don't even ask about how many car-related outings we've gone on.)
My sister recently shared a website explaining that so many autistic kids have had Thomas the Tank Engine books, videos, and toys as their "special interest" that it's actually been documented in scientific literature. See this link: http://www.myfavoritetoys.com/autism_thomas.php
The neat thing about the article is that it describes how this fascination can be used in a therapeutic way.
And don't get me started on all the Aspy kids fixated on Harry Potter...