It seems like it's necessary to have more than one reader on a Windows machine for .epub files. I started out using Adobe Digital Editions exclusively, but after I ran into some weird formatting and generally wonky behavior in ADE, I installed Bibliovore. I have to alternate between the two readers, because some files will display correctly in ADE, while others display correctly only in Bibliovore. It seems like the creators of these files pay little attention to how they'll behave in Windows, since they're intended for tablet-style devices.

Sometimes, I'll try to launch a file with Blibliovore, it will start to load, then the app will say 'Book Unavailable' and close on its own.

I don't want to have to keep loading new readers onto my laptop. Does anyone here know of a more reliable reader? I know there's an .epub plugin for MS Edge but I refuse to use Edge or Internet Explorer, ever.

There is a freeware program called Calibre that does e-book management. I use the Mac version, but it is available for Windows too. It comes with an .epub reader that works just fine. Calibre will convert e-books from pretty much any format into pretty much any format as well.

I do not use Windows, but the Kindle Reader app is free. Give it a try, it might work for you.

I have a Kindle, so I just use the app to send .epub files to the Kindle.

I've used Calibre on my PC to successfully convert several book formats for use on either my Android devices or Kobo reader.

Caliber is great for any e-document management.

Lets you convert and transfer across almost any platform.

Calibre is what I always use on PC. However, that was partly because I needed to be able to edit ePub files, which wasn’t as common of a feature. Never had any issues with it so far.

- Bret

I've been using SumatraPDF for epub & PDFs.

https://www.sumatrapdfreader.org/free-pdf-reader

Calibre to convert to AZW3. This seems to work better than MOBI. My reading is done on a Kindle Oasis - remarkable device.

I just use the reader that comes up after I borrow an ebook title that only comes in epub.

I use Calibre as well. It's a pretty good program. I don't even pay attention to what format my ebooks come in anymore, Calibre seamlessly converts when I send it to my device, and I've never had a book that didn't just work. I had a number of Nook ereaders, and now I have an Onyx Boox. Calibre has worked flawlessly with all of them.