Publishing? There are big companies or now you can self-publish. There are many video tutorials, courses, and blogs all dedicated to self-publishing, so even the online education realm of things seems to be taken.
Writing books? Ok sure you can write a book just like you can produce a single piece of any kind of content, so that can't count. Software to assist in writing books? From my quick search, there's Scrivener, and a bunch of other platforms and apps.
Want to write a book without typing? Hmm, there's already a bunch of speech-to-text apps like Dragon.
Buying books? Amazon. Audio books? Audible.
Collecting books? Hmm ok, may be something in there. Site that lists the top 100 books from prior years? Or a top 10 per genre, per year (historical).
The reality is coming up with an idea is already hard. Coming up with a business is much harder. Would love an idea in this space.