As suggested at a previous post it might be good to have some offline data stowed away.
May it be in form of a prepared mobile device, a pen drive or a hard disk. Some even suggest Raspberry Pi setups, which make it easy to serve/share data in a hotspot WiFi — aiming more at TEOTWAWKI situations.
One thing is for sure: having a (encrypted) pen drive with essential proof (IDs, licenses certificates, etc.) should be the bare minimum. As pointed out below you want to considered long-term storage like an M-DISC or swap the drive on a regular basis. You also need to update your outdated files, so you might want to do this in one step.
The topic and different approaches were often discussed at reddit and I'll try to give an overview and animate people to also post good digital reading materials and sources.
Wikipedia
We already mentioned the offline wikipedia with KiWix and ZIM-archived, browsable data:
Maps
You might also want to store map data with
Coming from https://www.reddit.com/r/preppers/comments/oxbna2/prepper_hard_drive/ someone posted the http://survivorlibrary.com as good starting point, and an article shtfblog.com discusses a bug out library on SD/USB drives.
I will edit this post when I find the other sources in my archives and would be glad to incorporate your suggestions.
Edit
I have been skimming through several sub reddits like /r/preppers in search for a collection I once saw. However, you could also use Google and search for yourself.
Many of the sources are unchecked for quality.
⚠️ Some resources have an American touch and might address gun smithing, general warfare, military SOPs and field manuals as well as civilian sheltering and or resistance stuff.... because of, well.. it's an important thingy oversees. Sieving through these collections is an absolute must.
Collections
Medical Books