Came across something whilst looking through the CityPrepping site. It's a software project called Kiwix. Site is here https://kiwix.org/en/
It's a software app for most platforms. What it does is enable you to download complete archives of things like Wikipedia, WikiBooks, and loads of other resources. You can then access them from your local machine, as you would wikipedia, but no need for internet access. As well as wikis, there are things like medical information, lots of technical stuff etc.
Now, these archives are big - wikipedia is over 100Gb. But you can save the archives (zim files) to any storage. I'm currently downloading wikipedia to a NAS. There are also apps for ios and android.