Comment by GTP

I won't contribute links to resouces, as in this thread there are many users that have listed everything I would have, plus more. I'll just recommend the Kiwix app[0], which is a convenient way of downloading many of the resources suggested by others, and keep those handy. That is, assuming you have a way of keeping your smartphone charged ;)

[0] https://play.google.com/store/apps/details?id=org.kiwix.kiwi...