I know this is an old post by now but> Something I've been wondering is, what official stuff (such as manga) is there besides games?
It’s mostly just games and manga, though there’s some ebooks too.
If you want a recommendation I think Forbidden Scrollery (pic rel) is by far the best touhou manga, and it’s definitely the one that focuses most on world building if that’s what you’re looking for. It also doesnt really require familiarity with the games. None of the manga really do, except Silent Sinner In Blue for which you really need to play 6 7 and 8 before reading.
Check out moriyashrine.org they have links to all the official stuff that’s been released. Figuring out what order to read everything might be tricky, but honestly that sense of piecing things together is just kind of part of the touhou experience.