Gerda malaperis (you may have to visit link in new tab)
A strategy I like to use when learning languages is transcribing down a beginner's book. It probably doesn't work for everyone, but it works for me. I did this with Gerda malaperis, a wonderful little book to learn Esperanto, converting it into an epub, since I didn't see one available on the internet. I used the official website, copy pasted the html into a text file, did a little processing magic with python to format it nicely, and converted with pandoc.
Gerda malaperis website
Copy-pasted Gerda malaperis
Python script to convert
To run the script:
python3 convert_gerda.py pandoc gerda_malaperis.txt -o gerda_malaperis.epub