Created Tuesday 27 December 2022
It is a given that printed books, etc. remain available in cases where you are not able to get online.
Books, guides, etc. that you have downloaded in PDF format will remain available to you even if you are offline.
However, an extremely useful resource is Wikipedia, which is online by its nature. You can have and use Wikipedia (or rather a snapshot of it) using a tool called Kiwix, and by downloading a snapshot in the form of a .zim file.
Be aware: .zim files of Wikipedia can be quite large. As of this writing, it is 89 GiB in one file. A slimmed down version without pictures is 13 GiB.
Kiwix is available for Linux, Windows, Mac, Android, iPhone, and also as a browser extension for Chrome, Firefox, Edge and others. There is also a Kiwix server package that can allow you to stand up a replica of Wikipedia or other .zim files and serve them over Wi-Fi or wired LAN.
Note also that any of the .zim files can be downloaded via BitTorrent by adding .torrent to the end of the URL.