> If not please suggest me gnome app in which I can open epub files.
https://johnfactotum.github.io/foliate/ (assuming by "gnome app" you mean an app that runs on gnome and fits in reasonably well).
Foliate is a great reader for epub files and Evince for pdfs. You might have Evince already installed since it's the default on Gnome/GTK environments.
I read on many devices without discrimination, laptops desktops phones and kindles of course. It can be a useful something to do if you're "waiting" for something else or just want to read, but at that device.
The pros are that you can read anywhere, even on large screens and small screens, if you set yourself up right. By that I mean, I usually try to use software that allows 'immersion', going full screen, dark background, and no controls visible. Just the book, and continuous scrolling.
On laptop/desktop, I have tried a lot of ebook readers, and the best one I've come across is Foliate.
For phones there's Moon Reader which lets you customize the visual options.
The cons are that for reading many hours, it's not as good as an eink screen. Glossy screens are the worst, matte screens are good, but eink is best. Also, it's easy to get distracted and switch to something else.
What part of Kindle? The physical ebook reader device, the book store, or just software to read ebooks?
There's plenty of the latter in the open source space; with any standard GNU/Linux installed on any kind of device, you could have a folder on disk full of .pdf or .epub or whatever files and read them with the likes of GNOME Books or Foliate.
I've seen some raspberry pi projects people had built with an e-ink display if you want a physical reader running free software, there may be some easier options for that, or get any tablet computer capable of running Linux on it (MS Surface Pro, etc.); if you want the Kindle book store or things that get closer and closer to Amazon's proprietary products, you'll likely not find much luck there.
Thank you!
Something that should be noted is that Calibre could be used in conjunction with SumatraPDF. Use Calibre for library management, Sumatra as your Windows Default EPUB/CBZ/DJVU/PDF reader. It will work much faster than Calibre's built-in molasses-slow reader.
There's also Foliate, but it's still Linux only. Windows builds are possible (other GTK programs have done it) but still ongoing research.
I use Calibre and Foliate (Linux) on my laptop.
> the way epub format is designed, you can not annotate it.
https://web.hypothes.is/demos/epubjs/ is one example of epub annotation. https://johnfactotum.github.io/foliate/ is a desktop reader (for Linux) that supports it. Jorkens supports annotations on Windows. There are probably many others.