I use a Kindle Scribe. The iPhone can share any .txt, .html, .epub, .PDF content from the web to Kindle App so I can sync on the Kindle Scribe and read offline later.

I read all my Rustdoc and mdBook documentation this way. Even the ones I made.

The writing experience on Kindle Scribe is pretty good as well. I only use it for quick marginal notes and underlines. Sometimes drawing diagrams.

But I used it mostly for reading.