HI, actually autistic person here who's special interest is actually super Mario (and particularly Luigi), I'd like to point out that while he enjoys reading, I wouldn't say it's a special interest. A special interest is something huge and also personal, it's like a special thing that makes your entire brain light up and feel good to be involved with! And while Luigi does enjoy reading, he isn't exactly getting into the nitty gritty of, say, book binding, or certain authors, or anything broader about the concept of reading and the hobby. I think the presence of books is more there to imply he's more of a nerd, smarter than Mario. Nothing wrong with saying it COULD be a special interest, but just enjoying something isn't a special interest! The machinery/engineering/mechanical thing is definitely an interest of his I agree.
Aaah, I gotcha 👍
That I can definitely get behind. Especially since we get to see the interior of Luigi's Rank D house at the beginning of Luigi's Mansion 2 and see just how much he customized it. The masonry on his fireplace has this fun wrench-plunger emblem ingrained onto it, and his stairs have similarly wrench-themed railings installed. I'd bet money Luigi crafted those additions with his own two hands.
And of course, there's the enormous bookshelf in the corner. His reading habit might not qualify as a special interest, but it's definitely a beloved hobby.