On Sun, 6 Jan 2019 16:46:52 -0000 (UTC), arlen holder
wrote:

What is a decent general-purpose free epub reader, that, hopefully, if
possible, is open source & works on Windows, Linux, Android, & iOS?

I'm using "Cool Reader" on Windoze and Android, works very well and I
like the features particularly on an Android tablet.

https://sourceforge.net/projects/crengine/