I use Comic Zeal. It is not connected to or proprietary to a given store, which suits my needs. CBR and CBZ files are merely RAR and Zip archives with a renamed extension. Comic Zeal will accept a folder of sequentially numbered image files, as well. (No need to archive.)
Most ebook formats do not support images all that well. (Size limitations.) So I use Comic Zeal to fill that gap. I've digitally archived some of my print books, such as old Starlog PhotoGuide Books that were starting to show signs of yellowing. There are also archives on-line where one can download old STARLOG magazines and articles — but I imagine you have your own sources.