Kindle paperwhite and best offline resources

I've been studying since January. I will be offline for about 70 days starting in July. I would like to at least maintain what I have learned so far.

So far I have found minimal resources that work well on the kindle paperwhite – one grammar book is ok – but most just don't render well. Some graded readers and/or bilevel books would be great also. Any recommendations gor beginnerclevel materials on the kindle?

I will have my phone but power management will be an issue. I use Anki.
What would your study routine look like if you only had a your phone available in airplane mode?

Thanks