>>13328092G3L here.
As far as I know, in all 3 countries there's local dialects of German that exist alongside Standard German (which is the official language used in government publications, the news, and that's most likely to be used by fiction writers). Non-standard dialects are mutually intelligible amongst each other for people from nearby regions, but it'd be hard to find people from Northern Germany who perfectly understand people from Tyrol, even though it's not like it's impossible to learn to understand different dialects the same way you can learn to understand closley related languages.
There's lots of documentaries and reports you can watch on Youtube or download from
Archive.org for free, but I'm not sure if you're into that.
Aside from that, there's lots of poems, short stories, novels, etc. from the 18th-early 20th centuries you can read for free on Project Gutenberg DE and Wikisource. They have outdated spelling and some slightly outdated words, but nothing that can't be looked up in a regular German dictionary or on Google.
There's plenty of contemporary German fiction too, but I've never read any, so I can't give any suggestions on them or where to find them.
