There are cheaper options, but I love my phone sized Boox Palma 2. It runs android on a midrange spec chip, so you can do pretty much whatever you want with it, including Libby and old libraries that might be locked to an app.

If you want to go smaller and the other direction on functionality, the Xteink X4 is cheap and really interesting. It runs on a hackable esp32 and will magsafe to the back of your phone.